It wouldn't work at all, because units also revert to their firstshape at the end of every turn, regardless of whether they've been in a battle (they do it at the end of battle as well, so they can go through several firstshape steps if they are involved in battles). So by this method the gladiator would die after one turn.
I think the #onebattlespell thing is a good idea (if I do say so myself
). I will flesh it out when I get a chance. There is lots of potential to do cool stuff with this kind of thing.