Groudon raid

But he also doesn't resist it. Gyarados is usually good not only because water is SE against Groudon, but also because he resists ground/fire moves. Against DT/SoB, Gyarados drops way down the list and grass is far and away your best option.

https://www.pokebattler.com/raids/defenders/GROUDON/cp/51968/attackers/attackerLevels/30/strategies/CINEMATIC_ATTACK_WHEN_POSSIBLE/DEFENSE_RANDOM_MC?sort=OVERALL&weatherCondition=NO_WEATHER&dodgeStrategy=DODGE_REACTION_TIME

For unkown moveset and extreme weather, Ho-Oh with E/S is the top rated counter for Groudon. It resists all of Groudon's charge moves and deals a lot of damage in return.

With unknown moveset, it falls second to Gyarados in rain, second to Dragonite in wind, and second to Articuno in snow.

With extreme weather, it falls second to either Exeggutor or Venosaur against Solar Beam, second or third to Vaporeon and Gyarados against Fire Blast, and dropping its lowest to third or fifth against Earthquake.

There is always something better if you know exactly what moveset it has, but Ho-Oh is the highest overall ranked pokemon across all movesets and weather.

Or are you saying it wouldn't be a waste of resources?