The Imp is a rare monster type that only appears in select dungeons. It has low stats, but blinks to a random tile every time you strike it, which can deny you an easy kill if you haven't reached the place it blinks to.
You can prevent Imp from blinking if you
- knock the enemy into a wall (with PISORF, Bear Mace or any other source of knockback)
- push another enemy into it via PISORF (it will still teleport if you push the Imp into another monster)
- make monster Slowed with WONAFYT, WEYTWUT or The Earthmother's Entanglement boon.
They also don't teleport when you ignite burning stacks by attacking something else, but that fact generally works against you.
If you can pump both your health and mana into hurting it you can easily kill imps 1-2 levels above you. It may be worth the gamble to attack them even if you can't prevent the teleport, especially if you can poison it first. You can tell where imps teleport to by watching the animation after you hit it, so you might be able to chase it down to finish it off.