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Give it Hidden Power Ice or Grass, then. Hidden Power Ice would be generally more usable since it hits Flygon that otherwise sponges anything Ampharos throws at it. As I said, Thunder Wave goes against what a Trick Room team should be doing; being slower.
Sorry, didn't quite understand that. Sure you only need one Pokémon to get Trick Room up, but what happens when that Pokémon faints? The team is ruined. Keeping the Trick Roomers alive is key to the team working. As you say, Water Spout and Eruption need to come in almost for free. For that reason, you can't have the Trick Room users sharing the same weaknesses as the sweepers, else you'll never get them.
But he wouldn't be using Water/Ice, he'd be using Water/Ice/Grass, which has far more coverage than Water/Grass/Poison. Namely, it hits Dragons, but also gives you a better time against Flying types too.
Full Power Water Spout vs. Standard SubPetaya Empoleon - 37% - 43.8%
This is without any boosts from items. Therefore, if Octillery can manage to not do minimum damage to on both hits (one on the switch in and one on the next turn when it outspeeds), Empoleon will not be able to set up a substitute.
Charge Beam is 50 power whereas STAB max power Water Spout is 225. Even against Empoleon, Water Spout is going to do more. (50x2 = 100, 225/2 = 112.5)
but remember to factor in charge beam's 70% spa raise, although it gives free switchins on a choice set. also water spout damage is invariant. it's maximum every single time.
Oh, and you might want to consider a Physical sweeper instead of Camerupt. Aggron can work wonders due to Head Smash having no recoil on Aggron. Rampardos can work too but it can NEVER switch in and lacks the ability to use Head Smash.
my personal favorite TR physical sweeper is machamp, but YMMV.
Personally, I love Marowak as a TR sweeper. With Thick Club, EVs in attack, and an Adamant nature, it achieves the highest attack in the entire game - it surpasses even that of Attack Deoxys. You may want to run a moveset of Stone Edge, Earthquake, Fire Punch, and Double-Edge (due to Rock Head), but you could drop an attack for Swords Dance if you want further boosting (not sure if it's even needed, though!).