I'll just add some things that haven't been mentioned.
Trick a Choice Scarf on Clefable and it's crippled, it usually gets stuck in boosts and is forced to switch, and it's not going to be threatening your team without boosts.
Just use Taunt.
Roar, Whirlwind, Haze, etc.
Just be a good player and take a gamble, predicting is part of the game. Let's say Clefable has just come in on your Garchomp. 31% of the time they are going to go straight for Moonblast, and 69% of the time they will choose to set up in your face, expecting you to switch fearing Moonblast. But you use Swords Dance. Bam. OHKO with Iron Head. Even if the opponent did go for Moonblast, Garchomp can survive one from the standard Clefable at full HP.
Hit it. Hard.
There are these things called wall breakers. Just hit it hard with powerful moves, Fire Blast in the sun from Mega Char Y is a 2HKO, even after a Calm Mind boost. Talonflame can take a +3 Moonblast, although it will die from the BB recoil.
252 SpA Mega Charizard Y Fire Blast vs. +1 252 HP / 0 SpD Clefable in Sun: 229-270 (58.1 - 68.5%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Choice Band Talonflame Brave Bird vs. 252 HP / 252+ Def Clefable: 207-244 (52.5 - 61.9%) -- 98.8% chance to 2HKO after Leftovers recovery