LAST EDITED ON Aug-29-13 AT 05:38 AM (Pacific)Fighters are tanky because they have lower AC than every one else except for ninjas and monks whose AC get real low with ninjutsu. They also have superior hit points.
The other advantage that fighers have is a low experience requirement to gain levels.
So here are the forces at work here that determine the damage you take:
1) AC - I don't think AC does any damage mitigation, it only determines whether or not your character gets hit or not hit (misses or non-penetration).
2) HP - When a hit makes contact, the heavier your health pool, the more contacted hits you can take
3) Level - Also like AC this determines your chances to get hit or penetrated. I *think* this may also involve a a form of mitigation, where a contact hit that would do 15 damage may be mitigated down to 10 damage. I don't know enough about the inward mechanics to know that or not, but I only mentioned this as I've noticed that when my party vastly out-levels a group of baddies, my party will hit them for more than the normal range for their weapon opposed to hitting similarly leveled baddies.
So, if you don't have access to better gear, and your fighter is getting a beat down, your solution is simple: do some grinding and level up. He's a fighter, so levels should come pretty easy and quick. Even if his gear is pretty meh, the added levels pad his AC and of course he'll have more HP. Edit: When I say "pad" I don't mean that his AC will naturally lower, it'll still be the same, I just mean that the better the level ratio you are to your enemy, the less likely you are to be hit, even with the same AC.
If you don't want to or otherwise refuse to grind, the only other suggestion I have is to slap a shield on him to lower his AC and just parry each round. That is if your other characters are pumping out good enough damage to make that a viable option, it may not be if said fighter is also your main hack slasher.