All the DPS should have some level of evasive ability.
Think cinematically. The light fighter types should have to move around and dodge. Caster types can have some short distance teleport, healers can have some magical shield or contingent healing.
Just think of, say, a dragon. Huge paws, 2 wings, tail, firebreath. You will have to run out of the attacks, in real time. It's not going to be stupid numbers shit like "defense rating = % chance to be struck". You move, or you get fucking creamed. If you get clipped, you take heavy damage (it's a car-sized talon, for crying out loud), and that's where healing spells come in.
The entire fight should be hectic, unpredictable, and frankly, exhilarating. It shouldn't be a tank and spank down from 100% to 0% with a few horrible scripted events every X% hp dealt.