My Slaking was a pretty much destroyer in the Battle Maison in ORAS, I loved using him.
Slaking ♀ @ Choice Band
Nature: Naughty (+Attack -Sp. Defense)
-Retaliate (STAB, 170 base power if used after fainted teammate, 100 accuracy)
-Ice Punch (good power, makes use of Slaking's strong attack, deals with Dragon, Flying, Ground and Grass types)
-Shadow Claw (good power, 100 accuracy, used to deal with Ghost / Psychic types)
-Hammer Arm (Awesome power, used to deal with Rock / Steel types that resist Retaliate)
You might replace Ice Punch with something like Hyper Beam. I would still highly recommend of choosing Giga Impact over Hyper Beam though because Slaking's base Attack stat is litterely 65 points higher that it's Special Attack, plus both of them gain STAB. And the recharge would probably not even affect because it will be loafing around on the recharge turn.
Always remember that, because Slaking can learn these moves, Focus Punch and SolarBeam wouldn't work because of Truant. Slaking would be loafing around on the turn that you fire these moves off. The only way to let these moves work is 1) Use a Power Herb, 2) set up Sunny Day for SolarBeam or 3) Skill Swap or Gastro Acid Slaking's abilty away in Double Battles.
Just a tip:
If you have a Smeargle with Skill Swap and Entrainment, you can remove Slaking's Truant and give it to one of the enemy Pokémon's, wich would be pretty damn annoying for your oppontent. Still keep in mind that Smeargle still keeps the Skill-Swapped-Away Truant.
Hope I helped. :D