By 'outer' two layers, you mean the top and bottom rows, right? An interesting effect, but you may want to specify that the movements are largely randomized - also:
Starts moving cards in the order they would attack. If there are no slots, free, then the cards just wont be moved...Ive been contemplating making it so that they receive 3 damage if not moved.
How would this work against creatures that are already located in the top and bottom rows? Would they be shuffled to the top/bottom randomly? Or would they take damage automatically since they cannot be moved into the middle?
Yes Top and bottom rows is where they would be moved.
Units already in the top and bottom row would be unaffected in no way.
no units are moved to the middle row. If there are no spaces available in the top or bottom row then the card in the middle will stay in the middle.
Position is randomized when a card is moved to the top and bottom row.
Updated OP