How can I reliably increase our giant ape's AC to 16 (or better)?

Question

A tactical problem to solve based on last night's session

Tactical Situation

1. Wizard polymorphs warlock (8th level) into a Giant Ape. (Did T-Rex last time, looking for variety and higher INT (7 vs 2); we also like that Giant Ape has a ranged option, throwing rocks, which the T-Rex does not).

2. Giant ape engages with a platoon of hobgoblin cavalry (11 hobgoblins on 11 worgs, wizard refrains from using fireball for in-story reasons)

3. In two rounds, partly due to martial advantage, the AC 12 Giant Ape takes 87 damage (out of 157) while only engaging three or four of the enemy (the Barbarian and Fighter were also engaged and the "quantity has a quality all its own" phenomenon of 5e was in evidence)

4. Fight ended when bard (hidden) cast Wall of Thorns from his Anstruth Harp which broke up the hobgoblin formation considerably.

Problem: Giant Ape's HP melted down too quickly due to low AC

The AC of 12 didn't do much to stop hits from landing; we anticipate much tougher foes than Hobgoblins in the near future. (Party is made up of level 8 PCs). We are kicking around a solution that goes something like this:

1. Warlock is polymorphed into Giant Ape

2. Someone casts barkskin on Warlock, sets AC to 16

   Duration: Concentration, up to 1 hour
   You touch a willing creature. Until the spell ends, the target’s skin has a rough, bark-like appearance, and the target’s AC can’t be less than 16, regardless of what kind of armor it is wearing.

We believe that if we cast barkskin on the Warlock before the polymorph, the 16 AC would not carry over. (If we have that wrong, please explain why).

What we seek in an answer

Presuming that we have three eligible spell casters - Arcane Trickster, Bard(Valor), and Wizard(Evoker); warlock is the party's go to for being turned into creatures) - how do I get one of them Barkskin to cast onto her? {It's a druid/ranger spell, and the Anstruth Harp doesn't have the spell on it the way the MacFuirmidh Cittern does}.

Another good kind of answer

If there is a different way to boost the Giant Ape's AC, that's also a good answer.

Note: the group of players, as a whole, really enjoyed having King Kong (well, Queen Kong) mix it up during the fight. Assume for any answer that we will use the Giant Ape again, not a T Rex or other beast, since it was a very popular choice once we all did the after battle confab.

Answer 1

Any spell that can boost armor will work.

There's no real problem with casting barkskin or mage armor prior to transforming; the spell effect will carry over after you transform. Polymorph doesn't block any ongoing spell effects. While your statistics get replaced by the creature's, spell effects aren't your statistics, they're the effects of a magic spell and consequently carry over to your new form.

This is explicitly covered in the Sage Advice Compendium:

Can a creature under the effects of polymorph have other spell effects on them, or are those game statistics also replaced by those of the beast form?

Polymorph replaces only the target's character sheet or stat block with the stat block of the chosen form. Other effects, such as other spells, still exist.

So the easiest thing is probably mage armor (or in your case, the armor of shadows invocation), which should get your Giant Ape form up to AC 15, which is nearly your target. I don't know of a good way to get barkskin with the group you're with, but I think there's an easier answer on your own invocation list.

Alternatively, another player could cast mage armor on you -- it used to be "self only" in prior editions, but these days it can be delivered by touch. You may need to be a little thoughtful about how your gear is set up, though -- if you were wearing armor in your human form and then polymorphed into an ape, your ape form would be a valid target for mage armor but the spell would end as soon as you reverted because you'd then be wearing physical armor, which breaks the spell.

Answer 2

What you can do now

Magical Armor

As others have suggested, AC enhancement spells like mage armor and barkskin will remain in effect even after you are polymorphed (although for mage armor the AC will be based on the ape's dexterity).

From Sage Advice:

Can a creature under the effects of polymorph have other spell effects on them, or are those game statistics also replaced by the those of the beast form? Polymorph replaces only the target’s character sheet or stat block with the stat block of the chosen form. Other effects, such as other spells, still exist.

See also: Can polymorph end spells that require the target to be humanoid?

These may be cast before or after the polymorph, and if they were already in effect on the warlock they will be retained.

Dodge

Have the ape engage, and then take the dodge action. While at disadvantage to be hit, the ape's armor class is roughly equivalent to 17, and it still gets to make one opportunity attack on any opponents that leave its reach. This does limit your offensive capability, but allows the ape to distract and pin down the enemy while the rest of the party is executing their actions.

Consider other forms

The huge giant crab has better hp than the ape and AC15. As CR8 it should be available to your Level 8 warlock. However, only one attack, no missile attacks, and terrible Int score of 1 (even worse than the T. rex).

Ask your DM about animals off the books. If your DM is open to the wizard making up fantastic beasts based on rumors rather than experience, ask to work with them on a CR8 form that has a better AC than the giant ape.

For the Future

Shadow of Moil

When your Warlock hits ninth and can add / switch spells, she could add Shadow of Moil from XGtE.

Flame-like shadows wreathe your body until the spell ends, causing you to become heavily obscured to others. The shadows turn dim light within 10 feet of you into darkness, and bright light in the same area to dim light. Until the spell ends, you have resistance to radiant damage. In addition, whenever a creature within 10 feet of you hits you with an attack, the shadows lash out at that creature, dealing it 2d8 necrotic damage.

With a duration of one minute, this would have to be cast on herself just before she was polymorphed. However, once in place you can use the ape's low AC and high hp to great effect to winnow lots of chaff. In effect, you want the ape to be hit to get multiple necrotic attacks per round.

Unfortunately it would use the warlock's Concentration and would thus be vulnerable to going down on each hit, but with the ape she would at least have a +4 Con save bonus.

This would also stack with other armor solutions (mage armor, barkskin), including her own armor of shadows if she swapped that back in at 9th.

Answer 3

Ok, hear me out. Magic armors +1, +2, +3 don't require attunement.

Additionally, per the DMG in the "Wearing and wielding items" page 140:

In most cases, a magic item that's meant to be worn can fit a creature regardless of size or build.

I know "most cases" is vague on purpose, but assuming polymorph lasts one hour, and it takes 10 minutes to don heavy armor, a giant ape could don a magic chain mail, splint, or plate armor for 21 AC max if it's a +3 Plate.

I feel a DM might not allow these shenanigans, specially since an ape is not a humanoid. But if Hill Giants can wear armor, it's not a stretch for a big monkey to do so. If this was some sort of Kong vs. Godzilla situation, as a DM I would allow it.

Answer 4

For wild shape you can get an ioun stone and/or animated shield. Standard animated shield is just +2AC and ioun stone +1, but there is no reason +1-+3 animated shields or ioun stones can't exist. In the game I'm running the druid has a +1 animated shield for +3AC. Unfortunately in your case thoguh polymorph is worded so this won't work.

Barkskin should carry over though. Stats are replaced with the apes, but active spells don't go away as far as I know. So I think barkskin should keep functioning. Instead of a human or whatever race he is with barkskin, he becomes an ape with barkskin. That's part of what makes moon druids good, they can shape change as a bonus action, so they can cast a spell then shape change first round, and the spell effect stays on (like call lightning). So they can rain lightning bolts down on the enemy while having a bunch of temporary HP. If this works I don't see why barkskin wouldn't. This question is for wildshape but the same reasoning should apply to polymorph:

Does Barkskin cast before Wild Shaping apply to your beast form?

Answer 5

College of Valor, Combat Inspiration may boost AC enough

At the party's level, 8, the Valor Bard has 4 or 5 Bardic Inspiration uses per short rest. (Charisma of 18 or 20 assumed¹). While this won't stop critical hits, and neither will an AC of 16, rolling a d8 as a reaction to an attack will improve the Armor Class by an average of 4.5 for some of the attacks made against the Giant Ape. In those cases, the AC will range from 13 - 20 based on the Ape's Dexterity of 14 (+2 to AC) and the die roll.
Combat Inspiration

... when an attack roll is made against the creature, it can use its reaction to roll the Bardic Inspiration Die and add ithe number rolled to its AC against one attack after seeing the roll but before knowing if it hits or misses" (PHB, College of Valor)

Costs, Benefits, and Reliability

1. Costs: this approach has its {opportunity} costs . There is no free lunch.

   • Dedicating this party resource to boosting the Ape's chance to-not-be-hit costs the chance of another party member getting a hit, succeeding on an ability check, or making a saving throw with a Bardic Inspiration roll on the d8. The party needs to assess how often and how well they use Bardic Inspiration in general - since it recharges on each short rest this could be used to avoid taking damage via boosted AC about a dozen times per adventuring day, depending on how many short rests happen in a given adventure day.
   • This approach puts a burden on the bard player to be alert to this requirement for some combats (when Queen Kong is present) and not others, and to more carefully manage and apply Bardic Inspiration than he does now. (Yes, I am in the group, and I am self-answering). In a party where the Bard player is very pro-active, this is not such a burden.
   • If the Giant Ape uses a reaction to avoid a hit, that reaction during that round can't be used for an opportunity attack (if, for example, bard casts fear or dissonant whispers, or enemy tries to flee), and thus a chance for another 3d10+6 bludgeoning damage is foregone.

2. Benefits: compared to getting access to Barkskin

   • this resource is rechargeable, costs no gold, no consumable items and costs no spell slots.
   • AC versus a given attack can exceed 16 (averages 16.5) - it can sometimes exceed the stated goal, up to AC 20.

3. Reliability: not airtight.

   • On a roll of 1 through 3 it fails to meet the goal of AC 16
   • This isn't an always-on AC boost like choosing the Armor of Shadows invocation, per @DarthPseudonym's answer, or the barkskin that lasts as long as the polymorph spell lasts: 1 hour.

What's useful in this answer?

1. While this may not solve the problem completely, this approach applies assets already under the party's control, and may be a suitable temporary solution until the Warlock hits 9th level and can add the Armor of Shadows invocation for an always-on AC of 15 (per @DarthPseudonym's answer).
2. This {self} answer has the virtue of letting the reader see egg all over Korvin's face 🤣 The fact that we have a College of Valor bard in the party has been true for over a year (joined when another PC retired) but none of us has ever used "boost your own AC" or "boost your damage" feature from Bardic Inspiration so far; it is usually applied to an attack roll or a saving throw by most of the players. The answer was Hiding In Plain Sight.

¹ _{Just checked on roll20 char sheet, the bard's Cha is 20}