Anyway, your opponent was sub par, full special attack team, including Cloyster. No experienced player would have given you the chance to set up so much.
How to counter?
- Any move that eliminates stat changes. Seriously that screws it up. Haze. Etc.
- Taunt. Use Taunt. It's screwed. Eg. Taunt Gliscor. Taunt prevents Cosmic power and softboiled, Ground typing prevents Charge Beam, Stored Power is pathetic without boosts and you can tank it and kill slowly with E-Quake. Or a Dark Type, forcing it to revert to Charge Beam to do damage.
- Just bash it with powerful attacks.
>252+ Atk Choice Band Technician Scizor Bullet Punch vs. 252 HP / 252+ Def Clefable: 284-336 (72 - 85.2%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Technician Mega Scizor Bullet Punch vs. 252 HP / 252+ Def Clefable: 212-252 (53.8 - 63.9%) -- guaranteed 2HKO after Leftovers recovery
As you can see it isn't going to be taking many powerful attacks like that. And that's defensive Clefable. Lets look at Specially Defensive
>252+ Atk Technician Mega Scizor Bullet Punch vs. 252 HP / 4 Def Clefable: 308-366 (78.1 - 92.8%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Choice Band Technician Scizor Bullet Punch vs. 252 HP / 4 Def Clefable: 416-492 (105.5 - 124.8%) -- guaranteed OHKO
So hit it with powerful physical Steel and Poison STAB and watch it die~
Tell me more about how it's going to take on SD Talonflame, SD Aegislash, SD Lucario etc.
4 . Put it to sleep. That was Magic Guard variant, so Sleep will still affect it anyway
5. Boost alongside with it. If you get the same boosts as it does, you're back on level ground. Apart from Stored Power wrecking you.
6. If paired with Smeargle - take smeargle down. You should be able to. Smeargle generally carries sash, as a single half-decent hit will take it down to 1 HP, It'll probably use spore, switch out and hit it with a faster Pokemon. Smeargle dead. Now you can phaze out Clefable at will without any ingrain popping up.
7. Trick it any choice item while it's still boosting. Have fun Clefable, now you're stuck using Cosmic Power.