Use Corpse Eater on a Team member when they are bleeding out, or I guess after they have died. Never waited for that so I can't confirm that it works, should be easy to arrange if you don't get the chance in one of the tougher battles. Obviously you do want to reload the last save after doing this.
It's Bormir and Farris that can have Corpse Eater mutation, Keep in mind that you need to equip it in your inventory if you have unlocked multiple minor mutations and it isn't selected. This need to be done before the ambush starts.