A helpful note for everyone.
You *can* use the damage hack for Arena, but be careful!!! There is a right way and a wrong way of doing it.
1. Only use it *after* using your special move. I don't know why, but if you use the hack before doing so, you will one tap your opponents and your progress won't be recorded. Activating the special disables one-tapping for whatever reason.
2. Once you've done that, and I can't stress this enough, USE THE MOD SPARINGLY. That is, only turn it on for brief intervals. Let's say your opponent(s) have very high health levels and you can tell that you won't finish the fight in time. Turn it on, get a few big hits in, switch it off, and finish the fight without high damage. Rinse and repeat as necessary. That will work.
P.S. I tested it too, to see if I could get away with activating the super move, then leaving the hack enabled for the entire fight. No dice; I received the unusual activity message and my progress wasn't recorded.
Please note that you have to finish the arena fight to make it count. It used to be that if time ran out, the winner would be whomever caused the most damage, but that isn't the case anymore, unfortunately.