Mega Gardevoir is a pretty good wall-breaker. Pixilated Hyper Voices tear huge chunks of health outta a lot of stuff and ignore Substitute, Light Screen, and Reflect. She also gets Psyshock to slap special walls like Chansey around.
-Mega Gardevoir w/Gardevoirite
Nature: Timid
Ability: Trace--->Pixilate
EVs: 252 Sp.Attack, 252 Speed, 4 Defense
Moves: Hyper Voice, Psyshock, Focus Blast, Will-o-Wisp/Taunt/Calm Mind
Keldeo works fairly well too thanks to Secret Sword:
-Keldeo w/Life Orb
Nature: Timid
Ability: Justified
EVs: 252 Sp.Attack, 252 Speed, 4 HP
Moves: Hydro Pump/Scald, Secret Sword, Calm Mind, HP Flying/HP Ghost/Icy Wind
You can also do a Specs set on it but without Calm Mind.
Black Kyurem is another wall-breaker with it's good mixed offensive stats:
Kyurem-Black @ Life Orb
EVs: 252 Atk / 56 SpA / 200 Spe
Lonely Nature
- Dragon Claw
- Fusion Bolt
- Ice Beam
- Iron Head/Earth Power
There's probably others too, but I don't remember them all.
Also, I wouldn't really call Mega Metagross a wall-breaker. Base 145 Attack is certainly nice but it ain't cracking apart walls left and right either, especially since it lacks a (reliable) way to boost that Attack. Meteor Mash/Bullet Punch, Zen Headbutt, and Earthquake are practically mandatory on it, leaving you with only 1 slot to play around with, but regardless of what goes in there shit will still wall it. You lack Ice Punch and Lando-T and Gliscor wall it. No Thunder Punch or Grass Knot and Mega Slowbro's or most bulky Waters wall it (or at least give it a hard time). Not to mention it hates getting burned or paralyzed.