One opening that's gimmicky but effective if timed right is an opening with Weavile and Lucario.
Lucario has the Mega-Stone, but doesn't evolve on the first turn. It instead takes advantage of Justified, its hidden ability to get up to a +3 attack boost from Weavile's Beat Up, and then Mega Evolves on the next turn and murders everything with Earthquake or Rock Slide. It's probably best to also have a priority move on it. An example could be:
Lucario @ Lucarionite
Trait: Justified
EVs: 252 Atk / 252 Spd / 4 HP
-Rock Slide
-Earthquake
-Extremespeed
-Close Combat
Weavile @ Focus Sash
Trait: Pressure
EVs: 252 Atk / 252 Spd / 4 HP
-Beat Up
-Ice Punch
-Ice Shard
-Low Kick
I find that if you can get the timing right (it takes practice) you can destroy with ease. Obviously this is based on the original Terrakkion Beat Up set so credit to whoever made that. I just think that this shows the versatility of mega Evolution and how you can use the two abilities and boost in stats to maximum potential.