During Mission 4: Proxy War
, you'll regroup with the troops, and storm the base upon the missile strike, sprint through all the enemies to spot 2 helicopters flying in around the base including one coming in for a landing. As the helicopter lands, it will usually drop of 4 enemies, and if you throw the molotov at the group right as they jump off, it should ignite them and unlock the achievement. However, if you're having difficulty getting the chopper to spawn with a full squad, you should be able to run up the side, take it out, and then have a group of 4+ enemies attack you from the right, usually lining up for an easy molotov as shown in the video.
Additionally, you can reload the checkpoint and grab all 3 of these achievements in the same spot: