A great place to get this is in Tychos Ribs, at the first area you spawn in. Go through the second door and this is the situation:
-There is a spawning point on the right (a building) for at least three Lost Legion Eternals (which transform after taking 50% damage)
-2 Lost Legion soldiers (DO NOT KILL THEM!)
-1 Eridian to the left (a single one will spawn in the far back-right after the first dies; kill these guys)
-A Turret (DO NOT KILL IT!)
-There is also a Freeze Barrel near the middle, and an Explosive and Corrosive barrels by the LL Eternal spawn. (Take these out without injuring the standard soldiers!)
Now, you goal is to injure the Eternal to about half health, then let it transform. They randomly become one of several types, (Tempest, Frenzy, Phasewalk, and Magus). You ONLY want Magus. If the wrong one comes up, kill it and let a new one spawn (and if they run out, quit and reload).
Then stand near the two soldiers and let the Eternal Magus attack you with their AOE attacks, which will injure and eventually kill the soldiers. Finally, go by the Turret and do the same there for the final enemy to get the achievement!