Have we really not done this one?
While I love the concept of the Armorer, I think overall the reigning champion of most powerful Artificer subclasses is the Battle Smith. The main reason for this is the versatility of their weapon choices, which both allows you to spec into various weapon-type-focused feats as well as getting any kind of magical weapon that your DM grants you, rather than being limited to what you can replicate.
Also, while they're limited to Medium Armor, the fact of the matter is that if your Dexterity is 14 or higher (and I try to get that on any character who doesn't wear heavy armor) you'll only be a single point behind someone wearing plate armor, which, coupled with the Shield spell as one of your specialist spells, means you're going to be one of the hardest targets to actually land a hit on.
Battle Smiths also get a kind of mini-smite in Arcane Jolt that can also be used to help heal (while they're the best Artificer for pumping out single-target damage, they're also going to have a decent amount of supplementary healing. Not enough to replace a Cleric or Druid, but arguably on par or even better than most Paladins).
The Steel Defender pet is interesting: there's surely some shenanigans you can pull with them, though I'm going to be treating them largely as just a little bit of bonus damage and some protection with their reaction (the latter of which is not going to come up in my calculations here, actually).
But let's also figure out the following:
Martial characters rely a lot on getting good feats to boost their damage. But these feats typically only boost Strength or Dexterity (or your choice of each). All Artificers use Intelligence as their primary stat, with Battle Ready allowing you to use Intelligence for attack and damage rolls when wielding magic weapons. We can assume that you'll have magic weapons, because you can make them yourself with your Replicate Magic Item feature.
But do we pick up exclusively Intelligence-boosting feats (or even ASIs) at levels 4 and 8 because these improve our attack and damage with weapons, as well as our attacks with our Steel Defender, as well as our spell attack bonus and save DC? Or do we allow our Intelligence to languish for a bit in order to pick up something like Great Weapon Master, Polearm Master, or something else of that sort?
Here's my guess: the extra damage from a Great Weapon Master feat is ultimately going to be big enough that it's worth delaying capping our Intelligence. There will certainly be other ways to build this, but my gut tells me this is the best for prioritizing pure damage.
Taking this feat, we'll need to start off with a 13 in Strength, and this will put our Strength to 14. Is there some argument to be made for multiclassing into Fighter? Probably, but I'm not going to roll that way.
If we pick a background that lets us start off with a 17 in Intelligence, that will mean that we can get an Intelligence feat at level 8 (or we could get it at 4 if we're willing to wait for GWM until later) to push it to 18. Do we get two more such feats at levels 12 and 16 to cap Int before we get our Epic Boon at 19 (assuming the campaign even goes that high)? Maybe, or we could just take an ASI at that point to leave room for something else at level 16. But that's beyond the purview of this post. We should assume we're looking at an 18 Intelligence by level 10.
But what Intelligence feat are we picking?
My default is typically War Caster. It's just so good. But we do, admittedly, already have proficiency in Con saves, so it's not as crucial for protecting our concentration (though Con save proficiency and advantage on concentration saves synergize very nicely). Because Battle Smiths do not have the "Bladesinger extra attack" we're less likely to pick up cantrips like Booming Blade, though the "spell as opp attack" is certainly not limited to such cantrips.
If we're in an Eberron campaign, there are certainly some feats that tie into, like the Dragonmarked origins that can be nice (though sadly, House Lyrandar Heir as a background doesn't boost Intelligence, so we wouldn't be able to get its associated feats and get Greater Dragonmark to be able to cast Conjure Minor Elementals by 7th level. Amazing for a Monk, though) but I'm going to try to be setting-agnostic here, so we'll limit things for the time being to those feats found in the PHB and in Arcana Unleashed.
The latter is going to require a bit of thought, as I haven't yet really internalized what all the best feats are from that book. I really like the "Spell School Adept" feats, though these are of course most impressive on full casters. We also already have Shield, so that makes Abjuration Adept less exciting. Evocation Adept will give us Fireball. Conjuration Adept's boost aside from the extra spell preparations doesn't actually help Conjure Barrage, given that it helps with concentration saves, but it will give us Conjure Animals, which is actually a very good spell (it is, I believe, strictly better, or near enough, to Spirit Guardians at the same level). You know what, just because I always go War Caster, let's have some fun and pick up Conjuration Adept so we can run Conjure Animals. It might require some real flavoring to make it make sense, but it's good.
Actually, here's my pitch: We're a gnome (I usually leave out species choice here, but this is primarily for flavor) and all of our inventions use the time-honored power of Miniature Giant Space Hamsters running on wheels. When we cast this spell, we're unleashing a swarm of Miniature Giant Space Hamsters to gnaw at our foes.
Ok, so, our feats give us Great Weapon Master at level 4 and Conjuration Adept at 8. We'll be a little spread thin with Point Buy, but we should be able to get Dex and Con both to 14 if we get a background that can give us both Intelligence and either Dex or Con, while starting off with 13 Strength (14 by level 4) but this will require us to basically dump both Charisma and Wisdom. The former isn't so bad if we have someone else in the party that can do the talking, though Wisdom is a little rough.
Next up, we're going to talk replicated items.
Simply put, we're going to, by level 10, make ourselves a +2 Maul (or Greatsword, but without weapon masteries, this is mostly arbitrary) and a Repeating Heavy Crossbow. Both weapons have the Heavy property, and will thus benefit from the damage boost of Great Weapon Master, which will be a significant chunk of damage at these middle levels. (I would for sure flavor my Heavy Crossbow as something more like a pneumatic bolt-launcher or even railgun.) That means we'll have great weapons for both melee and range, and the Repeating version of the Heavy Crossbow lets us use Extra Attack with it even without Crossbow Expert.
We'll also make a Wand of Magic Missiles, not for us, but for our Homunculus Servant. One level later, we'll instead hand them our Spell-Storing Item with Conjure Barrage loaded into it, which they can use basically every turn in combat, but given the breakpoint we've picked for these tests, we're stuck giving them the wand. We won't give them Dragon's Breath because we'll want to have concentration for Conjure Animals.
We won't be using Shining Smite, because this requires concentration. Certainly something to do if we are out of 3rd level slots, but not in our "ideal" scenario here.
Now... I think that's all we need to figure out before we start calculating damage.
Oh, one last note: while we'll have that Repeating Crossbow, the Maul is going to do more damage overall, so we'll assume we're wading into melee. That said, against a swiftly-flying Wyvern that also hits pretty damn hard with its attacks, we very well might prefer to stick to ranged attacks. But if we've allowed for all the other melee builds to not worry about that, we'll assume we are fighting in melee here.
So: Game Plan:
Oh no.
I just re-read Conjure Animals. It's a save-for-none spell. This pretty significantly changes my intentions. Indeed, it throws a lot of this build into chaos. While a save-for-none isn't necessarily terrible, it does cut into its actual average damage per turn by a pretty wide margin.
So, not only do I think this means we're not taking Conjuration Adept as a feat, it also throws open the question of what spell we even want to concentrate on.
For feats, I think we'll just retreat back to the default of War Caster.
But for our concentration spell, I see a few potential options: we could cast Haste on ourselves (if we have an ally who hits harder with attacks, we might consider them, though I actually don't think there are many who would do more). Haste is, of course, always a risky spell given the stun that happens when it drops, but our AC will be boosted on top of its already high level (oh, by this level we can have +1 Half Plate and a Cloak of Protection) and we do have Con save proficiency. Alternatively, we could stick Dragon's Breath on our Steel Defender (though it won't be able to use it on that turn given that we also need a bonus action to command it). I'm going to say let's do Haste.
Ok, phew, with that fixed, let's actually get our game plan together.
We have our Homunculus Servant off at a safe distance with the Wand of Magic Missiles. They're just going to be firing that off at 1st level each turn, dealing 10.5 damage on average.
On turn one, we cast Haste on ourselves. We can then run up to the Wyvern (with our presumably 60-foot speed) and then use our Hasted action to attack with our Maul
Now: bit of a sidebar here: does the Hasted action benefit from Great Weapon Master? I would argue that it does, because it is "another action," much like a Fighter gets with Action Surge, and even if it limits what actions you can take, it still does refer to it as an additional Action.
We'll thus be making one attack with our +2 Maul on turn 1, and then on turns 2 onward, we'll be making three such attacks.
If any of these crit, that will trigger GWM's Hew, allowing us to make another maul attack with our bonus action (no GWM damage bonus on this one, though the entire attack is bonus damage from that feat). If we don't crit, we use a bonus action to have our Steel Defender attack instead.
Notably, if any attack crits, we'll also expend an Arcane Jolt. We only get 4 of these per long rest, so we'll reserve these for crits only. This includes if our Steel Defender crits. I'll assume as well that we're not going to crit 4 times in a single combat, so we should be able to use this whenever we do.
And that is, I think, it, so let's get a-calculating.
Homunculus Servant: thanks to Magic Missile auto-hitting, and the Homunculus requiring no action to command, we'll just assume it's expending a charge each turn to deal average damage of a 1st level MM, which is 10.5. There are only 7 charges, but we're happy to blow through them and potentially turn it into something else with Magic Item Tinker when we run out. But yeah, assume this 10.5 average every turn.
Maul and Steel Defender:
Ok, our hit bonus is +10 (4 from Intelligence, 4 from proficiency, and 2 from the weapon's magic bonus) and the maul hits for 2d6+10 when used as part of our action. So, 17 average damage on a hit, and 7 more on a crit. Except actually, using Arcane Jolt on a crit will add 2d6, which gets doubled to 4d6 because it's a crit, meaning 14 extra damage on a crit as well, so that bonus crit damage winds up being 21 more damage.
Note that we can only do Arcane Jolt once per turn, which further incentivizes us to save it for crits. The math might get hairy thanks to that, but let's see if we can figure it out.
Turn one, we're only going to get one Maul attack. So, with a +10 to hit, we land on a 4 or higher, or an 85% hit chance. 17x85% is 14.45. Then, our 5% crit chance adds 21 more damage (with Arcane Jolt), so we're looking at an additional 1.05 extra damage (from the crit alone). So, this attack will have an average damage of 15.5.
If we crit, we'll then Hew with Great Weapon Master. This still has a +10 to hit, but only hits for 2d6+6, or 13 average, with just 7 more on a crit (by definition, we've already used Arcane Jolt). So, 85%x13 is 11.05, plus 7x5%, or .35, giving us 11.4 with the Hew attack. But this happens only 5% of the time, so it's really .57 more damage.
If we don't crit, we're going to instead command our Steel Defender to attack. This guy, though, only has a +8 to hit, so it's only a 75% hit chance. They hit for 1d8+6, or 10.5, plus an extra 4.5 on a crit, except actually it's 18.5 more on a crit because we'd be using the unused Arcane Jolt on this. So, 10.5x75% is 7.875 and 5%x18.5 is .925, so we wind up with 8.8 damage. This scenario, though, is going to happen in the 95% of the time where we don't crit with our Maul, so it's actually 8.36.
Bringing this together, we wind up on our first turn with 34.93 damage.
But that's just our first turn:
Second Turn:
Things are going to get a bit more complicated. We basically have three attacks. The first attack is actually identical to the single one we got on turn 1, so that's 15.5 on average.
Now, the second attack works the following way: We will only Arcane Jolt if we do get a crit on this attack but also didn't get a crit on the previous attack. So, while the 17x85% part of our calculation remains the same (14.45,) we now need a variable crit damage bonus. 95% of the time it's going to be that 21, while 5% of the time it'll only by 7. So, 95%x21 is 19.95 and 5%x7 is .35, meaning that the crit bonus here is now 20.3. 20.3x5% is 1.015, so our average damage for this attack is going to be 15.465 (very close).
The third attack (our Hasted action) is going to now only get the Arcane Jolt damage if it crits but the previous two attacks didn't. So, we figure that there's a 95% chance not to crit on the first attack and a 95% chance not to on the second, meaning that there's a 90.25% chance we didn't crit on those two attacks. Thus, if we do crit on the Hasted attack, we're looking at 21 extra damage 90.25% of the time and 7 extra damage 9.75% of the time. 21x90.25% is 18.95ish and 7x9.75% is .6825, so our crit damage bonus here is about 19.63. So, once again, we start with that 14.45 and toss in 19.63x5%, which is about .98, giving us average damage here of 15.43 or thereabouts.
Now, the Hew Attack is going to deal the same amount of damage if we actually get it, so, 11.4 average. But the chance we get this is much higher now, as we've had three chances to crit. 95%x95%x95% is roughly 86%, so after three attacks, that means we've got about a 14% chance to have crit on one of them. So, 11.4x14% is 1.596.
Then, that 86% that nothing crit means we'll use our Steel Defender, which deals 8.8 on average if it gets an attack, but that's now happening 86% of the time, so we're looking at 7.568. I should be consistent, make that 7.57, and the Hew attack 1.6, rounded to the hundredths' place.
So!
On turn 2 onward, let's add it all up: with the Homunculus shooting off Magic Missiles, and then our 3 or potentially 4 maul attacks, with a Steel Defender coming in should we not crit at all, we're looking at 66.07 damage each round after the first.
I think that's very respectable.
Note that the damage does drop off a lot if the Wyvern takes flight - we will need to use the Heavy Crossbow, which has a lower hit chance (Repeating Shot only gives a +1 bonus to attacks and damage,) as well as lower damage dice, and then you also don't get to do your Hew attack. However, technically speaking, you can actually give the Wand of Magic Missiles to your Steel Defender and let your Homunculus Servant make ranged attacks on its own. Its range is a pitiful 30 feet, though, so good luck on getting it close enough to actually hit the thing. Still, you have options that, say, a Barbarian is unlikely to have.