I dont consider this really an exploit, but just a time saver, because once you imprisoned any enemy unit, you can kill him easily given preparation.
Ice Prison on enemy + Hildas Arrows....
The arrows should create mana/rage bubbles if they kill the enemy unit - now the game considers "killing the ice prison" as the same - meaning, the enemy below the ice unit will be replaced by a bubble.
Note that the ice shield is stronger for lvl3 and 4 units - so you have to damage it a bit - because hildas arrows doesnt do much damage.
You can basically kill 4 enemy troops at once with this if you have upgraded hildas arrows once.
Have fun
