The last boss is easiest with Infinite Ammo, Super Reload and Instagib Melee. Well all of them are really.
Infinite Ammo (Infinite Ammunition obviously.) - 100 Combat Engineer Ribbons.
Super Reload (Active reloads = 2x The Magazine Size. Works well with IA.) - Bronze Master-At-Arms.
Instagib Melee (Explosive Melee Attacks.) - 200 Wretch Kills in Beast.