Single Player: Kill at least 3 enemies with a single grenade
Your best chance to get this is a little later on in the 2nd level, Second Chance. First off make sure you have some grenades, so use you Visor to locate some ammo crates and see if they're holding any. Once you've obtained some, head down into the subway.
Work your way through it and you'll come to a gate you need to lift. First pre-load your grenades (double tap ) and then open the gate. A CELL unit will run out of the corner followed by some Swarm CEPH. Throw the grenade at his feet and let it kill all in front of you for the achievement to unlock!
Very easy spot to do this is the first time you get grenades on level 3 (counting the prologue). You will be on a roof and there are 3 guys standing straight ahead below. Go into cloak so no one sees you and cook the grenade a little and let it go. Bang!
Actually I should clarify, I just looked at the level select and the level is "Second Chance 02." It's weird how the levels are in this game. i assumed they were after a transition and an achievement but it seems that's not the case.
if you are doing it when you first get the grenades do not try and nit them beside the fire. tag them first and tag the guy standing over by the car too. then go into stealth mode and wait until the guys walk towards the car, the 3 guys will now be nice and close together. now is the time to hit the with a grenade