Random notes on the Doomsday matchup:
Super Nova: delayed dash under j2,b2 or MB b3 or MBDC. If you dash to avoid it, be aware that b1 xx black magic will NOT combo unless you j2 first. I'm not sure if this is a collision bug or whatever but yeah you can't punish with b1 in some cases like this.
Earth Shake: Block standing then quickly block low. I recommend pushblocking the first hit if you're cornered (but don't pushblock if Doomsday's trait is active). This move is around +0 possibly +1 on block but I'm not entirely sure. The MB version only hits low and launches. Can be hard to react to.
After pushblocking Doomsday out of the corner, it's still harsh to get out because you have to guess if he'll jump in Splash or go for his MB Venom (shoulder charge) to keep you cornered. If you decide to jump to avoid MB Venom, Body Splash will hit you. If you decide to dash to avoid the Body Splash, MB Venom will hit you. That being said if your opponent has no meter you really have no reason to fear Venom.
Again, don't pushblock Doomsday if his trait is active. Otherwise you'll waste 1 bar of meter because.. well it doesn't push him away at all.
Body Splash: Midscreen it's best to simply walk back or dash under and punish it's landing frames with a b2,3. If you're stuck in the corner you should ideally dash under on reaction to it but this can be hard to do online. D2 can sometimes be used to AA it (depends how soon or late he does the attack). You can combo with b1/b2 if it trades.
Venom: non-MB version is around -9 on block. You can punish it with 1,1 or d2. Combos I recommend: 11 xx T, b23, 11 xx black magic, dash, b23,u1+3 (43%) or: d2 xx T, b2, 11 xx black magic, dash, b23,u1+3 (29%) or: d2 xx T, b2, 22b1 xx MB black magic, dash, j3 xx far boot stomp (32%).
MB Venom: This is +frames on block so you can't press buttons here. Usual followup for DD is d1 xx Earth Shake so be ready to block that or tech throw. Try to blow low and react to the overheads. Although I admit I personally tend to block high most of the time because if DD sweeps I can at least roll/wake up and take minimal damage.
Trait: Black Magic, MB Lightning Strike, Boot Stomp and 113/22b1 are very good at slowing down Doomsday while his trait is active. Sometimes black magic causes massive pushback which is really good and can buy you some time as well as create good space.
Stage advantage: vs Doomsday you should pick a stage that allows you to escape the corner or use the corner to your advantage. One of my favorite stage counters vs DD is Hall of Justice - Great Hall because on the far right you have access to a free teleport after a pushblock. Same thing in Atlantis where you can MB both corner interactables to get Doomsday away from you.
4 matches vs DD:
www.youtube.com/watch?v=lfRDfJY9lmc
The biggest thing I failed at doing in those matches was punishing Doomsday's Venom properly. I only did d1 xx lightning storm when I could've punished with 11 xx T or d2 xx T for 30-40% every time.
The other thing was failing at activating Trait when I just needed some chip to win a round.
edit: Apparently I also failed at applying my own tech that I shared a few days ago lol. I completely forgot about Orbs 22 blowing up DD's MB Venom.