Here's a great sweeper moveset for Mega Ampharos.
Ampharos (M) @ Ampharosite
Modest Nature (+SpAtk, -Atk) OR, you could go with Quiet Nature (+SpAtk, - Speed) if you want to set up a Trick Room team! ^.~
- Focus Blast
- Dragon Pulse
- Power Gem
Thunderbolt is STAB (obviously).
Dragon Pulse is also a STAB move (when Mega-Evolved), it adds extra coverage, and it'll get the jump on Pokemon like Garchomp, which seem abundant in Gen VI.
Focus Blast covers Normal, Ice, Rock, Dark, and Steel types. It isn't the most accurate move, but it's powerful. (Base SpAtk of 120) As such, it has proven it's usefulness by getting me out of many tight scrapes in the past. (Especially against bulky Ice types)
Finally, Power Gem covers Fire, Ice, Flying, and Bug types. It is also more effective over a wider range of opponents than Signal Beam due to the fact that bug type moves are not very effective against Fire, Fighting, Poison, Flying, Ghost, Steel, AND Fairy types. This makes the move fairly useless online if your up against a Pokemon that is a mix of/is any of these types. Thus, Power Gem is the better option. =D