Everything except Malta sappers, radiation baddies like Nueron's baddies.
Have fun
Willpower's a bit of a generalist set. Praetorian Clockwork will stop your regeneration, so it might not be the easiest mob type to run through. Malta Sappers can be quite annoying. Carnies will be a little annoying with their on-death End drain, but (Dark) Ring Mistresses will be fairly painful thanks to Mask of Vitiation, a huge -regen debuff. Everything else will be fairly easy, so enjoy yourself.
As far as stuff that you'll really rip through, I'd say Council and 5th Column. Their robots are vulnerable to Smashing damage, which is what SS deals, and none of the rest of it will really screw you over.
I've seen Willpower tanks do naughty things to Rikti because of their almost complete reliance on Energy. It's hella easy to softcap energy defense on a WP tank.
ok so I have a level 50 wilpower super strength tank and I want to know if there is a certain group of enemies that it's really strong against? I know my psychic defense and resistance is really strong. So I thought about taking on some fortunatas in arachnos group. Does anyone else know any other groups? Help.