the best way i found (in my opinion) is to replay chapter 2: roadblock
just ignore the helicopter at the start and run to the end where all enemies are. the more damage you take while running there, the better. after the first group you go through a building. that's a checkpoint and you can quit and replay again.
mutagen i can suggest to help deal more damage:
samurai: because i am still working on the combat reflex chievo... and chances are you too ;) never take this mutagen off.
art of war: to increase damage.
blood rage: this is a MUST because at the start of the level, before you reach the first group, you let the helicopter damage you until your red bar is gone and only your organs are left. by doing this you'll deal much more damage and will speed up the killing process a lot.