My personal favourite Special wall in NU is actually assault vest Muk. Yeah it isn't a Grass type, but with Power-Up Punch, Gunk Shot, Ice Punch, and Shadow Sneak it can do some serious work.
As for Grass types that might work - how about Ludicolo? It is best used during the rain, but it still has some great potential.
Ludicolo @ Leftovers
Ability: Rain Dish/Swift Swum
Nature: Calm (+SDef, -Atk)
EVs: 252 HP/252 SAtk/4 SDef
- Giga Drain
- Hydro Pump/Ice Beam
Give it Leftovers and Giga Drain for healing, Hydro Pump or Ice Beam for coverage, Toxic for stalling, and have the last slot as Substitute.