This is a Metro 2033 achievement! Watchmen are the mutated hybrid wolf/rat like creatures you'll encounter in the SURFACE AREAS of the game, you can get plenty of kills while playing the level that I recommend you to:
DEAD CITY 1 & 2 (10-15 kills)
OUTPOST (5-10 kills)
ALLEY (20-30 kills)
TOWER (20-25 kills)
Remember that kills are cumulative through multiple playthroughs so you dont have to worry about it. Good luck!