If you're running a Focus Sash all of that bulk you're running is completely pointless. Invest all of it in speed. You won't be the best sweeper, but you'll out speed a lot of bulky threats that you'll probably be stuck 2KO'ing. And you won't have a guarantee that your opponent is going to hit you. Avalanche is just pointless negative priority here. Ramp isn't a tank, he's a pure glass cannon. ( You hit it and it shatters ) meaning you'll want to just go with standard Ice Punch here. ( Bolt-beam punches ) Now Zen Headbutt is so-so coverage, but there aren't many fighting types in NU, meaning that loses a LOT of appeal. Earthquake is amazing paired with Mold-breaker and also offers amazing coverage with the last move I'm suggesting.. Stone Edge! ( Or Rock Slide depending on preference )
It's STAB and Payback is now VERY useless considering you have a little bit of speed on ya.
I suggest removing it as it doesn't give much support for your team. Suggestions on replacements will be placed later on in my answer for this.
Take out Poison Jab for Substitute. It's literally giving you no support coverage wise and will pretty much always be out-shined by his other two moves. Substitute will offer you protection from being burned, paralyzed, or poisoned. While also supporting a few other things if you time it right. Item of choice would probably be Life orb here as well. Focus Sash + Sub is never a good combo ^^
I also suggest replacing this one! xD
Once again will be mentioned later.
Payback isn't offering much HP Dark couldn't give you, so I would switch those Attack EVs into HP or one of your defenses. Replace for HP Fire for the various bugs and opposing Amoonguss users ( He's apparently rather popular )
Lots of changes to be noted here. He can't make good use of Bulk up, so ditch it. Replace with Pursuit ( explained later ). Giga Impact should be replaced for Return / Frustration ( also explained later. ) Jolly > Adamant for several threats you need to out speed.
Yamask (F) @ Eviolite
EVs: 252 HP / 4 Def / 252 SDef
Calm Nature (+SDef, -Atk)
- Power Split
- Calm Mind
This heavily supports Slaking. Strategy goes like this. Opponent hits Yamask and gets Mummy. You switch into Slaking and use Pursuit. You now have Mummy with Slaking. No more truant! :D Now as a pokemon in generally he has Will-O-Wisp to cripple physical opponents, and Calm Mind to set up and annoy special opponents. Hex is good STAB when paired with Will-O-Wisp.
Basculin (M) @ Mystic Water
EVs: 4 HP / 252 Atk / 252 Spd
Adamant Nature (+Atk, -SAtk)
- Aqua Jet
Hazards and status abusers are annoying, so Taunt is there. Waterfall is your main STAB, while Aqua Jet is for priority abuse. Having 2 STAB moves is fine since one is priority, and he has that ability, and he has Mystic Water :P ( Grammar fail )
Crunch is nice coverage to use with Basculin. It's also pretty much the only good coverage he gets as a sweeper. He's a great sweeper to use without setting up though. You aren't running rocks meaning Charizard and Moltres can run all over you if they take out Ramp.