Personally I think your team should look something like this:
- Cacturne / (swap with Linoone as and when needed) / Filler
Aggron isn't that great because of how late it evolves, and its low speed and several weaknesses. And Slaking can't be counted on to reliably KO your opponents.
You need Manectric for the 6th and 8th gyms.
Heracross does admirably against the 7th gym, and 2 of the Elite 4, and the Champion.
Linoone is a great HM Slave, a staple for any ORAS team, and you can swap it out for any stronger Pokemon you catch (or even Groudon).
If you don't mind grinding a LOT, Salamence and Metagross are great, but they take far too long to evolve, and you can't access them easily.
Be careful with the Dragon E4 and try to get and Ice move on Swampert and a Fairy move on Gardevoir. The Ghost E4 shouldn't be a problem as long as you use your strongest neutral attacks, and the Champion falls to a strong Swampert, with Heracross and Manectric pitching in where needed.