Well im not sure if this would help or how far of a walk youd be willing to go on, but in chrysolia forest you can trade a mothim, i think, for a trapinch and ive used flygon in a few playthroughs on past computers, and i gotta say it does pretty good work in this gym if you have enough patience to set it up with dragon dance to wait out the trick room, using the other pokemon you have in the double battle as just a turn to heal the flygon, and then just earthquake everything. Beyond that, i dont have any particularly good advice.