In that case, why did you ignore dragon dance and waterfall? Gyarados can learn both without TMs. Also the Pokemon League uses 5 Pokemon that are weak to fighting, so that's effectively equivalent to one Elite 4 member.
Why are you putting so much weight on each Pokemon's usefulness against Cynthia's Garchomp? The Pokemon League has 25 other Pokemon, and the other boss battles have several dozen Pokemon. Even if Garchomp is the single most difficult opponent in the game, spending one slot on your team and 120,000 experience just to defeat Garchomp will help against Garchomp a lot less than it will hinder you in boss fights that are almost as difficult as Garchomp.