Tip One: Levels
Higher level Pokemon have higher attack power, so it makes sense to use them. Grind for levels with easier Pokemon and over time your Pokemon will be stronger. This is a good strategy if you only need that little bit of extra power to be able to defeat it.
Tip Two: Combos
Landing as many combos as possible is very necessary when you are trying to get past a tough Pokemon. Remember your Pokemons abilities and use them to your advantage. On each move, think carefully for how you can get good combos.
Tip Three: Powerups
Powerups can be really helpful at times, even though theyre a bit expensive. Checking in everyday is a fast way to get coins, but you can only do it once per day. If you're stuck, it might be a good idea to buy a powerup.
These are the tips I use when confronted with mega Pokemon, and even though theyre not the best tips, they could be helpful. Hope I helped!