Encounter: Showdown (Chapter 20).
The easiest place for this is the Showdown Encounter from chapter 20. Select it in the main menu and play on explorer difficulty. After killing the first 3 enemies of this area, 4 new ones will spawn close together on a rock. There is a rocket launcher sitting on one of the boxes here. Pick it up and shoot the 4 enemies after they jump down the rock. Aim at the enemy that carries a rocket launcher, he has the strongest armor.
This should easily kill all 4 of them. If you don't get it on your first try simply restart the checkpoint.