Try to take your time to decide on each move. I am always getting stuck with the megas as well and since there is no time limit it helps to try to figure out good ways to land combos. If you still can't beat him, you can practice on easier-to-defeat pokémon so you can level up your pokémon and practice getting combos. Also, try to catch some pokémon that are super effective and/or powerful. Once you're that far in the game you should have sableye's mega stone, which can be really helpful in this battle. If you really can't beat him, maybe you can try using powerups to help. They're a bit pricey but you can get more coins by checking in everyday and replaying old stages.