You can use Cloyster if you want. Cloyster's a good Pokemon. It's just that you need a different set that actually makes use of its ability. Yes, Cloyster has great defense and can learn Spikes and Rapid Spin, but its terrible HP and Sp.Defense and typing prevent it from spinning and laying down hazards. Cloyster is meant to sweep:
-Cloyster w/Focus Sash or King's Rock or something
Nature: Jolly/Naive
Ability: Skill Link
EVs: 252 Attack, 252 Speed, 4 HP
Moves: Shell Smash, Icicle Spear, Rock Blast, Razor Shell/Hydro Pump
That is how you Cloyster.
And like Dark Azelf said, if you're gonna use Garchomp's mega form, put Tyranitar in the team. If not, then don't use Garchomp's mega form. And follow his move suggestions.
Nah ンン
“No, I... I have to be strong. Everyone expects me to."