Complete the indicated task in the listed level to unlock the corresponding hidden Boss.

Gnezdo (Steel Beast): Get 25 Coup de grace attacks before 80 kills.
Gnezdo (High Sea Fortress): Get 40 melee kills before 80 kills.
Napad (Urban Chaos): Get 15 Counter Kills before 80 kills.
Strelat (Mining the Depths): Kill 70 enemies before 600 seconds.
Strelat (Catacombs): Get 20 Quick Shot Kills.
Ubistvo (Rail Yard): Get 25 Quick shot kills before 80 kills.