You receive an "Assassin" Medal for killing three enemies with stealth in TDM (including 3TDM, FFA, and Classic) and Objective games.
No matter which game type you choose, approach your target from behind and wait until the blue "stealth" icon appears, then press the melee (Square) button. You'll perform a stealth kill; three of those in a game yields an Assassin medal.
However, it's also possible to get these Medals playing Co-op Hunter Arena - but only when playing as a Villain. As such, the most convenient way to get these Medals is whilst performing Hunter boosting for Treasures.
Once again: it's only possible to get these Medals in Hunter when playing as a Villain.