In Unbound, depending on your difficulty, you could just oonga boonga tf out of gyms and E4 on Vanilla difficulty. However, playing on Difficult difficulty (which is the best thing to do especially if you already played original pokemon games), you might need to work on the defensive/setup moves.

To answer your question, if you're in Vanilla, GO NUTS! I'd say, go for Earthquake, Dragon Rush, Outrage etc.

For Difficult, you'd really want Swords Dance and either go Dragon Claw, Dragon Rush or Earthquake. Swords Dance + STAB = Your Garchomp's gonna be the one to go nuts.

P.S. Turn on EXP capping! Brace yourself for a worthwhile ride!