Going with StrangerDanger, here, this set looks promising with the also uncommon, SpecialGross set. Both would require a "+Atk/Spatk, -Spe" Nature to work and Hammer Arm is a great alternative to Explosion when dealing with Special Walls like Blissey, Snorlax, Cradily and Regice.
Something like:
MixGross
Nature - Quiet
EVs - 252 SpAtk / 44 Atk / 212 HP
Item - Life Orb / Expert Belt
Hammer Arm
Gyro Ball / Psychic / Meteor Mash
HP Fire
Grass Knot
EVs generate
354 HP / 316 Atk / 316 SpAtk.
When factoring in the
Life Orb you will have approx.
410 Atk and 410 SpAtk. You will be losing 35 HP per, every attack.
Hammer Arm replaces Explosion on standard SpecialGross sets, allowing him to 2HKO several Special Walls such as Cradily, Blissey, Snorlax and Regice. Grass Knot and HP Fire are a strong attacking combination, hitting many of Meta's common counters. Swampert, Donphan and Hippowdon are all OHKO'd by LO Grass Knot. HP Fire is a 2HKO on Skarm and Tangrowth.
The second slot is reserved for your STAB Move.
It's already low speed, coupled with Hammer Arm powers up Gyro Ball, allowing to hit upto 150 Base Power. The toss-up is between Gyro Ball and Psychic. Without Psychic, Weezing is bound to wall you, whilst Gyro Ball is generally, a better STAB Attack when dealing with common "Bulky" sweepers like Chomp and Mence. Also, it
can have a higher Base Power.
You are, however still walled by threats like Dusknoir and Spiritomb, pokemon that are fairly defensive on boths sides, pokes that also weaken Gyro Ball with their very low speed.
Hammer Arm VS Explosion
The option of Hammer Arm over Explosion is the main choice that'll differentiate this from a standard SpecialGross. Whereas, on a standard SpecialGross, Explosion is your only way to deal with Blissey and other special walls, Hammer Arm allows Meta to battle on, to another day.
However, there are several cases, where Explosion can be seen as a better option. SpecialGross can come in on a onced CM'd Cune, take a Surf, then retalliate with Explosion stopping the setup. MixGross doesn't share this luxury.
Similiar things can be said for most setup pokemon. MixGross can never OHKO Gyarados, unless it's carrying ThunderPunch or HP Electric, which leaves it easily walled by common counter. Again, Explosion is a good way to defuse any tense setup situation.
EDIT: Looking over this set, its move-combination is exactly the same as MixApe with Gyro Ball / Psychic over Nasty Plot.