Garchomp takes x4 damage from Ice Type moves, and x2 from Dragon
Moves, so it would be obvious to use either an ice type or a dragon
type.
Weavile is an ice type that can outspeed a Garchomp and
potentially KO it.
Floatzel is a water type that can learn ice fang, ice punch and
ice beam and it is also faster that Garchomp, so that can also K.O
it.