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.
Avoid damage
Use Doppelgangers and Dogs as living shields while chased. Use a Transmogifier to change bodies when missiles are homing in on you. They will lose track and fly into a wall.