RPGs and grenades will help a lot when trying to get this achievement. Luck will also play a big part when trying to get this achievement legitimately, but if you play the game regularly in large team games these opportunities should arrive soon enough.
The RPG is going to be your best bet, and you will likely find some chances on capture the flag games, when people rush your flag, staying too close together. Try to use the RPG a lot, it will make this achievement much easier to obtain.
One other rather cheeky way to get this is on free for all; if you see a couple of people engaging in a knife fight try to lob a grenade or two into the center of them and hope that you aimed well, then quickly look for someone else to kill.
Like most of the other achievements for this game it can be rather easily boosted. Just get 4 or more friends together and play a small free for all game. Few people play these games, so you should all be matched together on your own. Get your friends to stand together in a corner and chuck any grenades you have toward that corner.