Or any level one that learns leech seed. The only thing that really takes this down is weather or status. Give either of them a lum berry and you're good to go.
Flash Fire Ferrothorn
Haven't seen this personally but apparently it can be a monster. Have fun killing this..
Toxic Heal Staraptor
Give this thing toxic orb, facade and max out attack. That's all I need to say.
EDIT: as for the Flash Fire on Ferrothorn you could give Volt Absorb to a flying type or any ability that that makes a Pokemon immune to their weakness.
These are a few common abilities that I have seen or heard of. If you need more then ask :)