A good Sableye could be useful, as it wouldn't be affected by some of his best moves, but against Normal another fighting type can always come in handy. Also if you had an Aron normal type moves wouldn't be very effective against it.
My personal choice would be Sableye (Fake Out, Cut, Night Shade, Fury Swipes). Night Shade wouldn't be useful against this gym but would be useful later on.
You can get them in Granite Cave.
You can also get Makuhita in granite cave, which could serve as your other fighting type if that would be your preferred method. (I personally like very few fighting Pokemon). I would teach it Vital Throw, Fake out, Arm thrust and Sand Attack.
Aron can also be caught there and some good moves for it early level are Harden, Mudslap, Headbutt, Metal Claw.
Aron would be my second Choice.
Last time I did it I tried using two low level Pokemon against his Slaking, one of which was Sableye. I kept switching between the two of them, using Fake out as I brought them out (they both had it) so Slaking couldn't move then switching the out as he was having his move off because of his Truant ability. His Hp went down slowly but surely with my Pokemon not getting touched. The only problem was that they ran out of pp after a Super potion or two.
I hope this helps.