Doomsday, just learn to block. Most of his stuff is unsafe and the stuff that isn't you can just backdash out of. Same goes for his supernova, just forward dash out of it. As for Nightwing that's a different story lol
Every single char in this game has some sort of weakness. And from what I'm reading from the high level players more and more, backdash and movement seems to be very important
one of the main reason defense is widley cosnider hard is that takes a lot of memorization as well as reflexes. You need to be fully aware of your tools and your opponents and knowing the appropriated counter to your opponents tool. Most will be trial and errors.
