sounds harder than it is:
my idea is something like this:
when a person attacks you in range but misses the ac, a timer is triggered, penalizing their ac++. When the timer pops (2 seconds tops) the ac is restored back down to harder to hit level, ac--.
If a person gets a hit, then the penalty is ac+2 instead of just 1.
The idea is to model the concept that it is infact easier to hit someone when they are defending themselves already against a number of other attackers.
The benefit of this, is it gives some balance to really strong players, allowing the strong to be taken down easier by the many. It lets mid level people actually function as a benefit in a war, instead of being a simple garnish.
If the effect proves too pronounced, it can be compensated by only going into effect if the attacker came somewhat close to braking the AC...... and not at all if a lvl 1 alchemist is trying to punch a lvl 100 warrior in PMoP.
Another possibility is to use 0.2 or 0.3 shift in a total effect modifier number, which is converted to a whole number (so .4 would be 0 effect and .6 would be a 1) before getting added to the player's AC.
Bring out the mobs
