Yeah, that's what I found. Ironically enough, after going online with Supes, MMH, and Doomsday, I finally completed the challenge with my main--Joker. Pretty hilarious, considering most of his specials are so garbage outside combos or setups, and I didn't use Crowbar or Acid Blossom at all (didn't want to risk a botch BF3 or BF2 input wrecking my chances). So turns out yeah, you can't even perform a normal, but you can move around, block, etc. I don't know about trait, executing/teching throws, or pushblocks, but it seems safe to just stick with strictly specials. Meter-burning also is a bit tricky--I didn't meter burn a single move (again, I'm pretty amazed I actually did it) so I still don't know for sure.