When an enemy shoots you, hit him with the Soul-Swapper. After switching bodies, his own bullets will kill him.
Use a Transmogifier on an Alien or similar difficult enemies to transform them into something that may be easier to kill.
Activate a Bomb, but do not drop it. Use the Soul-Swap with an enemy and he will be holding it when it explodes.
Drop Drugs on your enemies until they die from overdosing.