If you like, you could give Lucario a try with the following set:
Lucario @ Lucarionite
Ability: Adaptability
EVs: 252 Atk / 4 HP / 252 Spd
Jolly Nature
- Close Combat
- Bullet Punch
- Crunch
- Swords Dance
I've used it in Showdown once, and it seems to work amazingly well. (: After a single use of Swords Dance, it could wipe out your opponent's entire team pretty easily. Either that, or you should probably go with Mega Aggron.
Aggron @ Aggronite
Ability: Filter
EVs: 252 SpDef / 252 Atk / 4 HP
Nature: Adamant
- Iron Head
- Curse
- Rest
- Sleep Talk
Not sure what Pokémon you should remove, though... perhaps Excadrill?
Excadrill is necessary for the team because it gives Volcarona and Dragonite Rapid Spin support, since they're weak to Stealth Rock.
I don't think Dragonite is a good fit for a Sand team because 1 round of sandstorm renders Multiscale useless.
Extreme Speed is also viable on Mega Lucario because of its +2 priority, and it also hits as hard as Adaptability-boosted Bullet Punch. Though Earthquake is redundant with Close Combat, it lets Lucario hit Aegislash without triggering the Attack drop of King's Shield.
Also, I don't agree with Curse as an option for Mega Aggron. Mega Aggron meant to sponge hits while striking back hard. Its Attack and Defense are also reasonably high enough to not rely on Curse:
-Stealth Rock
-Heavy Slam/Iron Head
-Thunder Wave
-Earthquake/Rock Slide/Superpower
Nature: Relaxed
EVs: 252 HP/4 Def/252 SDef
Item: Aggronite
Ability: Sturdy
or
-Rest
-Sleep Talk
-Heavy Slam/Iron Head
-Earthquake/Dragon Tail
Nature: Relaxed
EVs: 252 HP/4 Def/252 SDef
Item: Aggronite
Ability: Sturdy