If you're having problems with a particular stage go for a duel or survival round and play some in order to get coins. Then use those coins to upgrade your weapons, armor, and helmet and then go for the troublesome stage. You can only duel once every four hours so just forward the time on your phone in the time settings and you'll get your dueling privileges again. Body armor should be first on your shopping list as it will increase both your attack power and your defense.