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.