Struggle Bug Shuckle with maxed out Special Defense and HP, Chesto Berry and Rest. Chesto to instantly wake up from Rest, Struggle Bug to slowly lower Special Attack so it does less damage. Just remember to use PP Max on Rest so you can use it 8 times, not just 5. After that, battle of attrition. If Soul Wisp works, try that on a mon with Focus Sash so it doesn't immediately die to weaken it even further. Don't bother with Toxic, it doesn't work, trust me, I tried.