Usefulness of high AC at high levels (Tier 3 and 4)

Hi all,

I'm building a character at level 10 who will play through to level 20. I've heard that AC-focused builds become less effective at higher levels, but, looking at the Attack bonuses in the MM, it's not clear why that is. For instance, an ancient red dragon has a +17 attack. It seems like going from an AC of 18 (plate) to an AC of 25 (plate+shield+shield spell) would take one from being hit 95% of the time to 65% of the time, which adds up to a lot fewer hits. What am I missing? Is the idea that distinctions between ACs in the 13-18 range matter less at high levels, so acquiring heavy armor on its own is less of a big deal?

High level AC builds have three problems:

1. it's hard to raise your AC as fast as monster attack bonus increases; reducing hit chance from 40% to 30% is a 33% increase in toughness, reducing hit chance from 70% to 60% is a 17% increase.
2. Damage that doesn't go vs AC is increasingly common.
3. Your damage suffers significantly, because there are no damage-boosting feats for single weapon styles. Being hard to hit doesn't matter if the monsters just ignore you while beating on your less armored allies.

My recommendation: if you want to play an AC focused build, play a paladin -- they get a lot of their damage output from smites so the lower base damage of one handed weapons matters less, and they have aura of protection to deal with damage that doesn't go vs AC.

To be fair, Piercer does allow for rerolls and extra dice on crits, and if you're going sword and board a rapier is just as viable as a longsword even for STR builds and with Vex queues up advantage enough to make the crit bonus more applicable, especially on a pure Fighter who's making 3 attacks per action in tier 3. Shield Master can also help provide the setup for extra crit-fishing with its Bonus Action Shove.

Obviously you're still going to fall behind truly dedicated DPR builds, but that's how game balance works. I would say that solely looking at the high CR monsters isn't the best frame of reference, since most campaigns will also feature lots of mob fights or bosses with minions. That can be where high AC really shines, when you've got three punks swinging at you and mostly missing.

I am firmly of the opinion that chasing a high AC is counterproductive. No DM will ever run encounters where one PC can't be damaged. The higher your AC, the higher the to hit bonuses of opponents will get that the DM uses. If your AC gets too high, they'll just switch to non-attack roll means of neutralizing the PC.

At lower levels I explain it this way. The DM could throw orcs at the party, but if someone in the party has too high of an AC, they'll start using Fire Giants (and they hurt a lot more when they hit). The DM will ensure that encounters are always the level of threat that the DM intends, regardless of what your AC is.

Another problem with maximizing AC, is that when you put all your eggs into one basket, your other baskets are empty. I've seen a lot of players who maximized their ACs, then got bored and felt useless, because they were doing single digit damage while everyone else was doing 20-30pts a hit. What monster is going to attack the tin can that might be hard to hit, but is only capable of tickling the monster, when the monster can go after that pesky rogue who sneak attacks every round, or the one carrying the two handed sword (and the feats to use it).

Monsters are just like PCs, in that they will focus on the opponents that are the greatest threat to them. Someone with a super high AC, is not a threat - someone who does a lot of damage, is a threat.

I'm also a firm believer in playing what you find most fun. If you enjoy being the guy with the 25 AC, go for it. But if your goal is to 'never get hit' - that will never happen. No DM would ever allow that, because where would the challenge be? And just think of how difficult a situation your fellow party members will be in, if that super high AC character gets dominated and turns on the party.

Oh, and BTW - in those games i've been in where one person always tries to maximize their AC - guess who always dies first? Killing the monster before they can hurt you, especially at range, is almost always a better tactic than just high AC. The higher the monster's attack bonus, the more damage they do. Characters with super high ACs, don't usually have the HP to survive the hits from monsters that can hit those super high ACs.

In general rational targeting strategy in D&D is:

• Pick the target with the highest ratio of (threat)/(difficulty to kill).
• Focus fire them until dead.
• Repeat.

There are various reasons monsters (and players) might not use that tactic, such as lack of information or inability to focus fire, but the obvious turtle in plate armor with a shield is pretty easy to recognize, so 'ignore or CC' will invariably be high on the list of options.

On the other hand, narratively/gameplay-wise the point of having a tanky character is to take the hits, so plenty of DMs will play into that expectation as well. Thus I don't think enemy behavior makes a good point for or against, since it's extremely table dependent. Similarly, the degree to which a DM is going to actively inflate enemy stats to "keep pace" with a PC is probably going to vary a lot by table.

Thank you all, for your replies!

It looks like the main issues are that: damage is reduced, high-level enemies' damage bypasses AC, enemies just go after more squishy characters, and the marginal benefit in toughness decreases as attack bonuses go up faster.

I think these are all good points, but it was the last one I was most unsure about. First, in both cases, dropping 10% in the chance to be hit cuts out one hit out of 10. I guess the argument is that this is less useful when enemies still have a 60% chance of hitting you as getting hit 6 rather than 7 times is still pretty bad but 3 rather than 4 might make more of a difference. Which makes sense. But I just don't see that many enemies with super high attack bonuses I guess, so I was wondering how that worked out in actual play. I guess the answer is the other non-AC-targeting stuff is much bigger of a deal, and that makes sense too. So why give up damage for a more marginal gain? Though if an enemy has 400 hp and the difference between a greatsword and a shortsword is like 15-20/round, is that really going to matter that much either? Maybe it'd mow through minions a lot faster.

FWIW, my actual decision relates to whether or not to use a shield on a character with paladin levels, which would necessitate the warcaster feat when it would otherwise not be super helpful (he has resilient: con). The character isn't really built for two-weapon fighting, but between nick and divine favor and spirit shroud and the possibility of extra crits with which to smite, it might be worth it anyway. He needs a charisma-increasing feat, so he can't swap warcaster for piercer or GWM or something like that.

I too have wondered about this recently.

They came out with a new unarmored defense through a feat called Infernal Bulwark. By level 8, I have an AC of 17 rules as written. I start out with one of 18 with my armor and shield, but that gives disadvantage on stealth roles. I’m not sure how much the stealthy version of medium armor costs or how much AC it gives, but -1 AC for stealth seems like a good deal. I just worry how it will handle later play. 

What do you all think? Is it worth it?

www.dndbeyond.com/sheet-pdfs/Actionsparda_165040730.pdf

At higher levels, you are much more likely to run into enemies that force saving throws on you. Goblins shoot atrows and swing swords. A death tyrant has 10 eye beam attacks that all use savng throws.

So AC is still usefull, but enemies get more ways of bypassing ac. If youre going to tank at high levels, you probably want to be a paladin (plate armor for ac, and aura to boost all saves) or an armorer artificer (plate armor+shield spell+mirror image spell for ac, flash of genius for saving throws) i will say at high levels, especially level 14+, artificers get insane magic item replication stuff. You can build an elemental gem every long rest and summon an earth elemental every day. 

At higher levels, the monsters start getting highly intelligent so they might know to attack the party wizard. However, you will also run into dm's who start custom designing their encounters specifically to counter each player's biggest strength. 

Some dms subscribe to the notion of "shoot the monk" because monks can deflect attacks, and it lets them shine. Others take the path of find the monks biggest weakness and put that in every encounter. Its basically the "dm vs player" mentality

So it is something to keep an eye out for. If your dm is thwarting your build, its going to get old real fast.

But if your dm isnt actively trying to bypass your strengths ans explout your weaknesses, then high ac still has benefit. But at higher levels, saving throws will become more important.

Quote from DMoldschool >>

Thank you all, for your replies!

It looks like the main issues are that: damage is reduced, high-level enemies' damage bypasses AC, enemies just go after more squishy characters, and the marginal benefit in toughness decreases as attack bonuses go up faster.

I think these are all good points, but it was the last one I was most unsure about.

 The marginal benefit of AC depends on where your current AC is relative to typical monster attack bonuses. In general a shield build is capable of keeping up as long as the DM is willing to give out magic shields, it is very hard to keep up otherwise (on average attack bonus increases by 0.5 per CR and likely monsters to face have a CR range of 50-150% of level, so on average the AC 18 build at level 1 wants AC 28 at level 20 to gain similar marginal benefits).

From what I've seen.  High AC is useful mostly against minions. But as many previous people have said. Higher AC means less in damage. Now is this always true. But the vast majority of the time You deal less damage.

Now, what I would recommend for a high level, high AC build is an AC high enough to soak up minion attacks, but not the BBEG. This means that you still have space for high damage, but your not going to go down immediately because of damage from minions.