Although he lacks some traits Regigigas has, he has a Focus Punch coming off his massive base 160 attack. That alone makes him a huge threat in the current UnovaRPG metagame.
Sp. Attack: 95
Sp. Defense: 65
First off, Slaking must ALWAYS have Focus Punch. It doesn't matter if you want to play all-out offensive or if you want to make a tank out of him, Focus Punch is Slaking's most reliable attacking move. Second, you must always have some coverage for Ghost- and Psychic-Types who resist Focus Punch, and since Slaking lacks the elemental punches, he must use something else. This "something" is Shadow Claw.
Now, we've taken 2 out of the 4 move slots. The other moves depend on how you want to play. Want a wallbreaker? Give him the BoltBeam combo. Want even more physical power? Give him Giga Impact and/or Earthquake. Want to inflict status ailments? Toxic's at your disposal. Seriously, Slaking doesn't have the greatest movepool, but he's got what he needs in order to become an absolute beast.
The only things it doesn't have that lower its potential are a reliable recovery method and the elemental punches. The only recovery moves he has are Rest and Slack Off, and both moves are currently broken. That hinders him A LOT, making status ailments something to REALLY watch out for when playing Slaking, especially Paralysis. The lack of elemental punches also hurts him quite a bit, since he has to rely on special moves to get coverage, and 289 special attack is not bad, but it's really low compared to his beastly 419 physical attack.
Slaking is a major threat, but it's also got its disadvantages. His comparatively bad Sp. Def plus the lack of recovery and reliable coverage really hurt his ability to wreck things. But he's still a beast nonetheless. However, there ARE some Pokémon capable of taking on Slaking and pulling off a win. Here are some of those Pokémon and their respective movesets, so you can finally get rid of the annoying sloth:
-Up to choice (Recommended: Toxic, Explosion)
-Up to choice (Recommended: Toxic)
-Up to choice (Recommended: Any decent special move)
-Up to choice (Recommended: Depends on how you're playing)