I'd say Mega Aerodactyl, Mega Salamence, and Mega Swampert.
While not "bulky as heck" per se, he is INSANELY fast and has decent Attack, but Helping Hand would boost it to amazing levels. Here's a set I would run:
Aerodactyl @ Aerodactylite
Ability: Unnerve -> Tough Claws
252 Atk / 252 Spe / 4 HP
-Ice Fang / Aqua Tail (basically any filler move BUT Roost)
Aqua Tail has a decently high chance of OHKOing Mega T-tar if you run Aqua Tail.
Oh, and screw you, Bisharp. 252+ Atk Mega Aerodactyl Helping Hand Earthquake vs. 4 HP / 0 Def Bisharp: 276-326 (101.4 - 119.8%) -- guaranteed OHKO
Earthquake smashes Steel and Poison, and Sky Drop takes one foe out of the fight temporarily to get a Wish off if necessary.
Literally perfect. So much bulk. So much speed. SO MUCH POWER.
Salamence @ Salamencite
Ability: Intimidate -> Aerilate
4 Atk / 252 SpA / 252 Spe
-Dragon Tail / Hydro Pump / (another coverage move, kinda like M-Aero, I recommend a special attack)
Specially Based MixMence. Hyper Voice and Heat Wave hit so hard together, it's awesome! You need all the SpA you can get, as you're running off of Salamence's lower SpA.
252 SpA Aerilate Salamence Helping Hand Hyper Voice vs. 0 HP / 0 SpD Mega Kangaskhan: 204-240 (58.1 - 68.3%) -- guaranteed 2HKO
And you outspeed, so that's nice.
252 SpA Aerilate Salamence Helping Hand Hyper Voice vs. 252 HP / 0 SpD Landorus-T: 246-289 (64.3 - 75.6%) -- guaranteed 2HKO after Leftovers recovery
More outspeeding. Oh, and Intimidate doesn't even matter, cuz you're special!
Bwahaha. These are the two S-Ranked Pokemon in Smogon's ORAS Doubles Viability Ranking, and see how much Mega Salamence wrecks them.
4 Atk Salamence Helping Hand Earthquake vs. 248 HP / 0 Def Heatran: 428-504 (111.1 - 130.9%) -- guaranteed OHKO
252 SpA Salamence Helping Hand Heat Wave vs. 132 HP / 0 SpD Mega Mawile: 228-270 (83.2 - 98.5%) -- guaranteed 2HKO
Just a little prior damage and you net that OHKO!
252 SpA Salamence Helping Hand Hidden Power Grass vs. 252 HP / 0 SpD Rotom-W: 176-208 (57.8 - 68.4%) -- guaranteed 2HKO after Leftovers recovery
Dunno why you would run HP Grass, but just in case you get lucky with breeding and you're having big trouble with Rotom-W...
While not incredibly fast, it's rather bulky, and has the ability to set up its own rain quite easily.
Swampert @ Swampertite
Ability: Damp -> Swift Swim
EVs: 252 Atk / 252 Spe / 4 HP
-Rain Dance / Protect
The choice is yours between Rain Dance and Protect... They each have very important niches and I'll leave that to you. Calcs: (All using Mega Swampert's stats)
-1 252+ Atk Swampert Helping Hand Ice Punch vs. 252 HP / 252+ Def Landorus-T: 308-364 (80.6 - 95.2%) -- guaranteed 2HKO after Leftovers recovery
Not bad. Some prior damage and that thing's gone.
252+ Atk Swampert Helping Hand Earthquake vs. 132 HP / 0 Def Mega Mawile: 368-434 (134.3 - 158.3%) -- guaranteed OHKO
252+ Atk Swampert Helping Hand Ice Punch vs. 248 HP / 0 Def Togekiss: 312-368 (83.6 - 98.6%) -- guaranteed 2HKO after Leftovers recovery
Same thing as Lando-T. Some prior damage and Togekiss is gone.
252+ Atk Swampert Helping Hand Earthquake vs. 0 HP / 0 Def Metagross: 314-372 (104.3 - 123.5%) -- guaranteed OHKO
(Calculated with Mega's Defense stat)
Get that garbage outta here.
I may be adding more as I think of them. This is just a preliminary idea.
And Mega Steelix is my baby don't you dare call it ew ;-;
Also, why was it tough to EV Train Mega Sableye? You are horde training, aren't you?