Mutant includes all non-human enemies. You should get this in one play through, but if you don't, there are many places to grind it out. Remember that mutants have less health on survival difficulties.
Dark star is an excellent level to grind many achievements. Load it up and use the flamethrower on waves of enemies.
Cursed is slower, but the enemies come at you in small groups.
On hunter, you can grind mutant kills by reloading the one combat part of the level. Most of the enemies are behind grates, so they are easy targets.