Replace HP Fire with HP Water so you can go against Fire types. And replace Iron Tail with Focus Blast. Now here's the rest of the team for ORAS.
Your Sceptile is at his most vulnerable when an opponent goes with Ice, Fire, Poison, and Flying. So you need Water, Electric, Ice, Psychic, and Rock as a counter (Ice Beam is used mostly by water types).
Starmie: Surf, Ice Beam, Psychic, Dazzling Gleam
Magnezone: Thunderbolt, Flash Cannon, Lock-On, Thunder
Rhyperior: Earthquake, Stone Edge, Megahorn, Poison Jab
That covers Sceptile's weaknesses. I think you'd probably want a fire type, and you also need a Flyer. Check this out. Unfortunately, most fire types in Hoenn are kind of overrated in my opinion but try it anyway.
Camerupt: Earthquake, Rock Slide, Eruption, Flamethrower
Camerupt has the solid rock ability, which limits the damage from any moves it's weak against. Flamethrower gets STAB, and a fully healthy Camerupt is unstoppable with Eruption.
Now, the last Pokemon to decide on is a flier. Here's a good team.
Salamence: Draco Meteor, Fly, Crunch, Dragon Claw
Fly, Dragon Claw, and Crunch are all physical moves, so Draco Meteor's reduction in Special Attack doesn't affect these moves. I have Crunch in here as a response to Phoebe's ghost Pokemon.
There you have it. This team features Grass, Water, Dragon, Fighting, Ice, Psychic, Fairy, Electric, Steel, Rock, Ground, Bug, Poison, Fire, Flying, and Dark type moves. These sixteen types can score super effective hits on ANY Pokemon type, and any combination of types (I'm looking at you, Sableye)! I believe this is the best possible team for any Hoenn League challenge.