Garchomp does NOT learn Draco Meteor by level up or TM. It learns it by move tutor when it's friendship is maxed out with your trainer.
No, Garchomp can't learn Fly.
Garchomp can only learn Outrage if you breed it.
garchomp can naturaly learn giga impact, earthquake
it can't but it learns dragon rush at level 55 which is a good alternative or draco meteor from the move tutor if you'd prefer that
the best move is Draco meteor
He could learn Draco Meteor.
Garchomp can learn Draco Meteor because it is a satisfactory/powerful dragon type Pokemon.
No, Garchomp can't learn Fly.
Garchomp can only learn Outrage if you breed it.
garchomp can naturaly learn giga impact, earthquake
it can't but it learns dragon rush at level 55 which is a good alternative or draco meteor from the move tutor if you'd prefer that
the best move is Draco meteor
No
Garchomp cannot learn Giga Impact from leveling up in Pokemon Platinum. Garchomp can learn Giga Impact from the TM that teaches it which is TM68.
By breeding.
I think you are mistaking "meteor blast", for meteor mash, or Draco meteor. Arceus can't learn meteor mash, but can learn Draco meteor by leveling it while it's holding the Draco plate.
Yes, Rayquaza can learn Draco meteor.