Killing 3 enemies of the same type in a row, grants you a "trophy stack" (one for each type).
You deal additional 3% global damage for every stack.
Enemies of which trophy you have, deal 10% less dmg to you.
-Bosses always give trophies, and can give you multiple trophies.
-You need to kill 9(?) enemies in the row, if they show up in big numbers (like Boltthrower)