Just read about this in the official (and usually useless and abusive) Ubi forums.

But there's a nightmare mode you enter by typing nightmare in the console when the menu comes up at the beginning of the game. It simply increases the size of the blood spatter and tweaks the physics so the tangos go flying. It's good fun.

so

(1) start game
(2) when opening menu appears bring down console (~)
(3) type "nightmare" (no quotes)
(4) try tango hunt or somesuch (I don't know whether it works in multiplayer)