If your having trouble defeating the purple and white ghost use the following strategy;
after you have chosen fight from the menu have Ramdom choose fury and have Wendalla choose deadly 3. It works every time. The ghost never have a chance because after Wendalla cast deadly 3 and she uses her two spells, Carnage and I think Undead maybe? You will be given an option to chose a character to finish it, chose Random to fight, after he finishes with Wendalla's combo, he will start his own with fury, that's 8 consecutive hits, works not just on the purple and white ghost but also on the toxic grannys too

just something I figured out while I was playing the game.