Also, Return > Hyper Beam since Hyper Beam prevents you from switching out during the Truant turn, whereas Return does not. Plus I'd hate to have Hyper Beam miss and end up not doing any damage for 3 turns...
you can add over 80 points to slaking's attack if you EV trained it and bred for a flawless IV, which is huge. those 80 points make the difference between using a great pokemon and an average pokemon.
and for the moveset, i think focus punch deserves a mention above aerial ace on pika's set.
If you have Emerald, you can use the Pick Up ability to get extra Earthquake TMs. They're rare, but they are there. I think your Pick Up pokemon has to be Lv. 91+.
Also, as aragornbird said (or implied), you should have Return on there and THEN consider adding Hyper Beam for the last move.