The unfortunate thing about Cloyster is if you invest in its (crappy) bulk, you don't have enough power. If you invest in power, it's frail as hell. It also really likes Shell Smash. Cloyster is one of those Pokemon that I think is better suited to Singles; once priority users are gone it can sweep stuff. So a better (Water type) Pokemon to tank Ice attacks for Mega Sceptile would be like Lapras Suicune or Assault Vest Azumarill or Wash Rotom or Alomomola or something.
As for Mega Metagross, it gains a lot of speed upon mega evolving and has fairly good bulk without investment. Something like:
-Metagross w/Metagrossite
Nature: Jolly
Ability: Clear Body--->Tough Claws
EVs: 252 Attack, 252 Speed, 4 HP
Moves: Meteor Mash, Zen Headbutt, Ice Punch, Earthquake/Hammer Arm
would be better.
Also Dragon Claw>Outrage on Garchomp. It's less powerful, but Outrage randomly selects its target in Doubles, which could mean that it'll hit the wrong Pokemon when you don't want it to, and not being able to switch moves when you need to sucks.
Nah ンン
“No, I... I have to be strong. Everyone expects me to."