The earliest you'll see C4 is in "Quartier Diamant", where you'll see a bunch of C4 next to you at the start. Select the redhead prior to this as you'll need to buy yourself some time while doing the achievement. Start out, grab the C4 and then head down the manhole near where you started. Head up/North into the top vents and continue until you reach the end and must turn right. Trigger the alarm and unlock the door in the next room, where you'll encounter the guard you'll need to blow a kiss at. From here, head North and into the first room on your left, then run to the far end and plant the C4 with to kill at least 3 of the now 5 people in the room.