Teach Sableye detect and rock smash, give it a Leppa berry, and alternate between the 2 moves so that Slaking can't touch Sableye until detect runs out of the Microsoft presentation editor. After lowering Slaking's physical defense a few times, Marshtomp or Hariyama should have an easier time finishing it off.
After defeating Norman, I recommend that you stop using Sableye and Aron. Both have low speed and special defense and will have trouble taking hits in the second half of the game.