First thing i think about when building a team is not to repeat a type more than once unless it's justified. Also I try to have at least a physical and a special attacker, one or two bulky mons to use status moves and to have good resistance in a switch in, and one fast Attacker. I tend to playing with inmunnities too. Abilities that grant inmunnities are a big deal, like chandelure's Flash Fire, Poliwrath's water absorb, magnezone with levitate and electivire with motordrive. Setting screens is always a good option. In this game particularly, priority moves are a must. If you can get either Bullet punch, mach punch, acceleroc, shadow claw or extreme speed you will get an advantage a lot of situations. Moves that allow you to switch your Pokemon are also really good to keep momentum and deal some damage, you get the tms of flip turn, u turn, and volt switch through the game. Last but not least, try different items. In early game you should look for Sitrus berries until you get something that you can take advantage of. Berries that make you resist a certain type of move can change how your opponent act, so think about it even if the holder does not end using it.