Either player could do it or the game was error when copy player data to arena list.

The idea is alter champ id to be different from exist champs, the game will always load fail when reading that data. 

Maybe i am wrong but i don`t think it was a game error.. like i said if i lost or skip the fight no error, game runs fine but if i won the fight then game freezes.. this make me think it was some kind of exploit .. as the player gets 2 wins and gets points.. Can someone from plarium look into this please?

When you fight a battle, it downloads data from the server, not on other player's device.

There could be something on that comp of your enemy and/or your comp against your enemy comp that is causing the bug. 

Plarium is already aware of this. There are repeated reports of this on their discord.