I just realized I forgot to mention this,
Garchomp wouldn’t be affected by the poison. Garchomp is a ground type, and poison isn’t very effective against ground types. Even then, Trainer could just use an antidote or a full restore. It would basically be the equivalent of using Acid which has a power of 40 and lowers special defense, which already doesn’t do much if your Pokémon isn’t a special attacker.
Torterra would still be affected because their a Grass and Ground type, it still wouldn’t be super effective, but it could still do decent damage.
Considering how I classified all of the dragons earlier with Pokémon types, If the first move doesn’t kill, Just Garchomp and Luxray could sweep the entire team (Let’s say Tsunami was like Cyrus’s Gyrados and knew the move Earthquake, I think Luxray’s defense is high enough that it could tank an earthquake). (With help from Staraptor if we are really gonna count nightwings as dark types)
The one challenge is Garchomp is also a dragon type, and considering the rest of them are dragon types (cause they are actually dragons), this is both an advantage and a weakness as dragon types are only super effective against other dragon types. In which case, at Garchomps current level, Garchimp could just hit dragon claw or dragon rush 5-10 times and be done with the battle.