Similar to Saz's answer, but this one can be used in Single Battle.
1) Be sure to take off Wi-Fi battle option if you're battling on PO, to keep the surprise effect.
2) Take a Yamask holding an Eject Button if you're playing in RU and under. Take a Cofagrigus with Eject Button if playing UU and higher.
3) Use a no-Truant abusing set for Slaking, but you must have Pursuit on him. Also, be sure to put Leftovers on him. Never use Life Orb, as he already has gargantuan Attack.
4) When you sense a Physical move incoming, switch into Yamask / Cofagrigus. The opponent's Pokemon will gain Mummy, and Eject Button will activate.
5) With Eject Button, switch in Slaking, who can use Pursuit to gain Mummy and therefore lose Truant.
6) Have fun sweeping.