When you complete the following task in the indicated level you will unlock the corresponding hidden Boss.

Napad (Urban Chaos)
Get 15 Counter Kills before 80 kills.

Strelat (Catacombs)
Get 20 Quick Shot Kills.

Ubistvo (Rail Yard)
Get 25 Quick shot kills before 80 kills.

Gnezdo (Steel Beast)
Get 25 Coup de grace attacks before 80 kills.

Gnezdo (High Sea Fortress)
Get 40 melee kills before 80 kills.

Strelat (Mining the Depths)
Kill 70 enemies before 600 seconds.