Easy enough trophy - just tranq 100 enemies with the M9 or the tranq sniper (or the tranq blade during the Arsenal Gear sequence...)
The numbers of enemies tranqed carry over on multiple playthroughs on the same save, so if you tranqed 50 on your first run through the game, you will need another 50 on the next run.
On the easier difficulties, shooting an enemy anywhere will knock him unconscious. On normal and above, shooting an enemy in the head or the crotch is an insta-K.O.
Questions, corrections and neg vote justifications below.