Very simple, easiest done against a group of enemies to maximise your chances.
Equip a rocket launcher and go into cover in front of a group of enemies. Hit LB to light a smoke and aim high to flick it away. They will start shooting at it straight away so just pop out of cover and fire a rocket into the group you should get at least 2 kills.
If your having problems killing 2 with one rocket just shoot them a couple of times first.
Can also be done with any upgraded weapon if your quick enough