One of the campaigns in which I’m playing has managed to step into a problem I’ve seen a lot since it was first brought to my attention years ago. A friend of mine was playing a character who couldn’t take very many hits before dying, and he realized that the best strategy was to not take hits at all. He raised his Armor Class to a ridiculous level, far beyond the capacity of the monsters he expected to fight, so that he could adventure without worrying about every stray arrow. The DM, however, looked at the character and asked that he pull back on his AC, reasoning that it made encounter design too difficult, either making that character too powerful or making the rest of the party too weak. That is, any monster that could reliably hit this character would be too good against other characters in the party.
As an example, let’s say his Armor Class was 55. A monster would need an attack bonus of +44 to hit him half the time on a primary attack, even higher to hit reliably on secondary and later attacks. A decent AC for a level 20 character can be around 45 (the highest AC among the level 18 characters in one of my campaigns was 41), so a monster that hits AC 55 half the time with a primary attack will hit a normal character half the time on a tertiary attack, with the primary attack almost a guaranteed hit. More hilariously, the feat Power Attack lets a monster reduce their attack bonus to gain a bonus to damage. A monster with an attack bonus of +44 can take a -10 penalty on attack rolls and still hit AC 45 half the time, but each hit will deal 20 extra damage. So any monster that can deal with AC 55 can ruin a character with AC 45.
This swings the other way too. A character with an especially low AC has the same problem in reverse, because a normal monster can expect to hit them most of the time. It’s a bit different because this is usually intentional. Characters who have low ACs are typically aware of it and choose low ACs because having a high AC requires them to sacrifice something else. Wizards have their spellcasting reduced for wearing armor, and the advantages of plate aren’t worth the penalties it gives. Rangers, rogues, and other classes also have some features that can’t be used in medium or heavy armor. Monks are right out. As long as they’re not begging to get stabbed, most characters are able to deal with having a somewhat low AC, and having a good front line helps a lot.
It also helps to have a DM who understands party roles. Even though it’s smart, it’s not very fun to have every monster blow through the characters in front of them to reach the wizard. Maybe after a round or two, most monsters will redirect efforts to the scariest characters, and it helps to have multiple monsters that can split the front line and the available healing, but in general a DM shouldn’t spend every fight leveraging the party’s greatest vulnerabilities.
The problem we have in our campaign actually hits all of these buttons for maximum hilarity. We have one character with a spectacularly low AC, a barbarian who put all of their energy into dealing damage. We have another character with a spectacularly high AC, largely to counteract the barbarian and draw attacks. When the second character, a healer, is really going, his AC is 20 higher than the barbarian’s. Further, the DM is sending monsters that are able to hit the healer, mostly because those are the only monsters able to stand up to more than one hit from the barbarian. So we have a situation where the monsters are drastically outpacing the damage of the party, and the only reason it hasn’t resulted in a few party wipes is because the DM isn’t using Power Attack out of the goodness of his heart.
We haven’t died yet, so there is a balance here, precarious though it may be. It’s also nobody’s fault, because everybody’s just doing the best they can to play their characters and balance the game, respectively. However, I get the impression that nobody’s really happy about it. The DM has mentioned a few times that the sheer damage output of the barbarian is making encounter design difficult, and missing one player on a 17 isn’t helping. The barbarian doesn’t like ending half of our fights unconscious, because time not playing is time, you know, not playing. Our wizard has given up on being missed and being near combat at all, and he’s spending his resources just making sure the barbarian stays alive. Even I, with what I thought was a decent AC, have gotten used to being hit more often than not. And all of us are feeling the pain of dealing with these high-level creatures, which limit everybody’s offensive usefulness (well, except for the cleric, whose offensive usefulness is a spiked gauntlet and harsh words).
The problem with a situation like this is that there are so many parts to it, so there isn’t a simple solution. If the barbarian tones down the damage, the monsters outpace us and we die, which isn’t fun. If the healer tones down the defenses, the monsters kill him and we die, which isn’t fun. If the DM tones down the monsters, we cleave through them with no effort, which isn’t fun. And if everybody changes at once, we’re not sure what we’ll lose in the characters or the general play experience. The only idea I can come up with is mixing up combat a bit, but we’ve created a wacky situation there too. Most monsters that target touch AC, bypassing the healer’s armor, are immune to my character as a whole. Most spellcasters can be shut down by our wizard before they can get off a single spell. And any group of monsters that we can deal with one at a time will have small, squishy individual monsters that can’t last a round with us. We’ve tried a few different ideas, but we always seem to end up back at the single giant enemy that can take a hit, deal tons of damage, and stand upright for four or five rounds.
Rather than a mark or some artificial defense bonus, the defense role in 3rd Edition parities is best served by the character able to gain the highest AC, so it’s not always the responsibility of other characters to keep up. But without 4th Edition’s well-controlled limits (read: stranglehold) on defenses and attacks, there’s the chance that a player can specialize in just about anything, creating a real good chance of breaking the game wide open. Whether it’s a spellcaster who can kill every enemy in the first round, the diplomat who can blink their way through every NPC interaction, or the half-man half-refrigerator capable of wading nonchalantly through half of D&D, the freedoms in 3rd Edition are balanced only by the players’ reluctance to exploit them.