My first advice would be Hidden Power Ice, however, it that is not possible, go with Flash Cannon. Ancient Power's added effect isn't worth the low base power.
A good moveset for a Trick Room activated Camerupt would be:
Ability: Solid Rock--------->Sheer Force
EVs: 252 Def/140Sp.A/116HP
- Earth Power
- HP Ice/Flash Cannon
Hope I helped.