You just need the right setup for this. It needs to be done in a mission that doesn't require Gabe as he does not have a required skill.
You need a Vanguard, scout, sniper, and heavy. Breached is a top right skill for the Vanguard. The other 3 shots are skills that can be unlocked immediately.
You need to find a group of wretches and Breach them. Then use a skill shot to kill one of the three wretches. Wretches are best for this as they are easiest to kill in one "shot" of the guns.