I've beaten Cynthia hundreds of times, with different sets of teams and Pokemons. At the top of my head, Pelipper takes down Garchomp the easiest of all. Cynthia’s Garchomp does not have a super-effective move for Pelipper.
My Modest Pelipper has the following moves:
- Air Cutter (super effective on Garchomp, also on Lucario and as well Roserade, and you guessed it, Gastrodon, if Diamond/Pearl, too)
- Ice Beam (also super effective on Garchomp, as well Roserade, and Togekiss, if Platinum, too)
However, I wont argue that Gyarados, also a same type Water/Fying Pokemon, is easily universally used everywhere and better choice overall to have in your team. Gyarados is an overused powerful sweeper Pokemon when used correctly with Dragon Dance, but takes longer to raise, slower to level up in the game. Wingull/Pelipper is easy to find in the wild and quick to raise.
Garchomp is a physical attacker and is defensively stronger against physical attacks than it is to special attacks.
Garchomp is a Physical attacker.
Pelipper is a Special attacker and has a stronger physical defense than Gyarados with a base physical defense at 100, Gyarados at 70.
Garchomp will outspeed both.
Pelipper will inflict more damage to Garchomp with special attack Ice beam than Gyarados’s Ice fang without swords dance or dragon dance.
Floatzel is also a good, underrated choice because of its speed and ice access, but is fragile at defense, and so is Weavile. Weavile is also a good addition to the team for Elite Four and easily sweeps Lucian’s psychic team before Cynthia. Both are not so versatile against majority of Cynthia's team as Pelipper is.
Specifically against Garchomp only and against Cynthia’s team, Ive always had no problem using Pelipper and lasted longer in the battle.