IMO you have two options:
1-) add a Mega-Salamence, before mega evolving he have intimidate and after mega evoling you can spam Hyper voice non-stop. You can add Fire blast to the mix but some people like to run a mixed set with EQ. Also protect is a must since M-Mence have a nifty 120 base speed that he can use to outspeed and kill some threats
2-) Counter Mega-Salamence, hard but doable. Priority are you main weapon, being offensive or deffensive (prankster), paralyze it, burn it, whenever to stop it.
Also, I may recomend to don't use choice items. VGC is very creative and being locked in one move is not always a nice option.
And check if Soul dew is allowed, if so with a single calm mind and soul dew latias can survive a hit and kill M-mence. Don't be afraid to use boosting move since it can take your opponent by surprise (no one likes to face a +2 mega scizor).
I didn't see anywhere that said Soul Dew was unauthorized, but I'll read through the translated site again to double check. Soft reset a few times to get myself a Modest. Probably not the beneficial Nature though. ...where do you even find Soul Dew? I think I got it in x/y, but I can't remember.
Is Hyper Voice Salamence a gen v move tutor thing? Haven't made it to the move tutor in ORAS yet. I was gonna try and get some pokes from 4chan and breed accordingly to get the pentagon thing, but seeing as how I have 9 or so days left til the event I might end up having to use the pokes I stated above. I've been getting real lazy about breeding and what not.
Probably run something like
Lead: Rotom-W/Choice Scarf - Trick, Hydro Pump, Thunderbolt, Protect
Lead: Togekiss/Sitrus Berry - Roost, Air Slash, Nasty Plot, Dazzling Gleam
Latias/Soul Dew - Recover, Calm Mind, Dragon Pulse, Mist Ball
Scizor/Scizorite - Bullet Punch, X-Scissor, Swords Dance, ??
Goodra/Binding Band - Infestation, Toxic, Protect, Fire Blast
Gliscor/Toxic Orb - Earthquake, Fling, Ice Fang, ??