fight the weakest enemys you can find. fight untils theres only one of them left( make sure its the weakest.) then attack maria until she on very low health(the reason i said make sure their weak enemys is so they dont k.o maria.).after the battle she should gain some extra HP. either this or just intense training. intense training would be a good idea apart from the fact that it takes ages and would make the game too easy but either would work fine.