So, the IDW TMNT series had a pretty credibly threatening Shredder. He was a badass warlord in Feudal Japan and managed to take over the criminal underworld of modern New York really quickly, fighting mutants, cyborgs and aliens in power armour as necessary. This is the Shredder that the one in Batman/TMNT is based on.

The only respect thread I could find for him is this one. Doesn't cover everything, though.

Anyway, what's your opinion on the toughest DC and/or Marvel character he could take out? I'd expect it to be a street-leveler since even this Shredder is a particularly tough street-leveler.