When you are doing the arena and are about to fight the Grand Champion, do his side quest first so that during the fight he does not try. When you are about to actually fight him, go into game options and set the difficulty to "Very Hard". This give everyone you fight more health. When you are attacking the Grand Champion, you can get a lot more free hits in without worrying about him trying to fight back. This works even better if you are using a very weak weapon. It is not about how much damage you do that increases your levels, but how much you attack.