At that point, there are only a few ways you can deal damage to it if you have no way of nullifying their ability.
It does not prevent confusion damage. It does not affect status moves, such as Tail Whip or Poison Powder. It does not prevent any indirect damage, such as from burn, weather, recoil, and entry hazards.
So the best thing to do would be to use Toxic on it and just wait for it to die. If they have Rest, you're in trouble though. The only way to beat them would be to use Taunt to prevent them from healing their status.