You'll need to wait until you've unlocked Sulphur Bomb and Comet Drop to accomplish this.
Get on the rooftops and find a group of enemies on ground level who are clustered together. Drug dealers are an ideal target as there tends to be often at least 5 of them standing in a small radius.
Lob a sulphur bomb at them with L1, as long as you have three clustered together coughing, then jump off the roof towards them, hold square to comet drop and hey presto, trophy time.
If you've got the upgrade to the sulphur bomb it expands the radius making this a bit easier, but that's not essential.