Hello i am the 4th gen dragon gym leader. A strategy i use when battling my challengers is to have a pokemon that knows reflect and light screen and use those two moves at the beginning of the battle. and then switch out to a dragon pokemon or any other type of pokemon, so that if the opponent uses a move thats super effective against the pokemon ur using the damage will not be severe to your pokemon.
P.S let the pokemon that knows the moves reflect and light screen hold a light clay so that the 2 moves will last for 8 turns instead of 5.
Hope u guys like this strategy and will use it for your future battles.