These are the top 5 I struggle against
Harley
Batman
Superman
Robin
Dead shot
Not enough match up exp because I've only played em once or twice but they worry me
Dr fate
Darkseid
Ivy
Redhood
Here are my general thoughts on these guys.
Batman: This matchup is hard. I would say that the matchup is clearly in Batman's favor. You can't zone him out because he can get in with bats, and once he is in, we don't have the type of wakeup that can contend with his pressure. Nor do we have a six frame attack to poke him out of his strings. Our walk speed helps in this one. The Batman players love the cross up fj3. If you can read this, you can actually walk in the opposite direction of the cross up and Batman will whiff. This allows for a full combo punish. The mb b3 is helpful as well. You have to be careful with that though...because Batman has a few quick hitting strings that will knock you out of it. When you get a knock down, attack. Batman doesn't have the greatest wake ups, and with our up close tools we can give him some trouble. You have to be willing to spend some bar in this one and make some reads. That is the nature of a matchup that isn't in our favor.
Harley: Harley isn't quite as bad. Harley has a jumping attack that contend with Lantern, which makes air to airs sort of scary. Her guns are quick enough to give us problems in the zoning war as well. In my experience, Harley is really about knowing what she does rather than what we can do. The Harley player is eventually going to go for the temper tantrum attack, which if blocked allows you time to get your stuff going. You have to sort of play with her in the mid range game. Good movement is key, as you can bait out a bad jump, or a bad attack, and go in with your b1.
Superman: This is another tough one because we sort of have to hold the f23 pressure. You can really frustrate a Superman player by stuffing their zoning. Remember you have a horizontal projectile. This means stuffing jumping lasers. The mb beam for us is effective, as it gets a knockdown and really turns the zoning in our favor. Baiting out wake ups is key here. You have to be able to punish the rising grab. With Superman you really just need to play fundamentally sound. Almost forgot! Superman players love crossing up after air dash. You can interrupt this with a fj1 and get a full combo!
Robin: Gotta zone this guy out. A lot of Robin players like to try to do that flip through the air move that I don't know the name of. They usually wont try it until they are in range to hit. Once you recognize this, stagger your zoning a little bit, so that when they eventually try this, you can mb b3 and get a full combo punish. You really have to know how to block Robin's strings and wait for your opportunity to attack when the battle is close. Careful of the string that has a second attack that is low. The footsies are not as much in our favor here because Robin has such long reach with his sword. If you can get a mb wall in, you will push him back full screen.
Deadshot: PATIENCE! I know it is hard, and in fact this is my least favorite matchup, (close with Batman). After Deadshot's b1 got the nerf we can win the up close battle much easier. Take advantage when you get in with our new found patch help. When you get a knock down Deadshot players love trying their knee wakeup, because they want to be back full screen. You should easily be able to predict this and punish with either a nj, bj, or a mb b3. Careful on the timing of the b3 though, the meter burn knee hits twice.
Fate: We can actually outzone him. When they activate trait, let them eat mb beam. They get antsy because they don't want to waste their trait, so you can get a few good shots in. When up close, always assume they are waking up with their push back ankh. This can be punished with a mb b3, or you can get close, crouch block, and get them with b1. Careful with his teleport move where he falls from the ceiling. You can full punish if you catch it. Just learn the movements for it.
Darkeid: You can fight the zoning battle, but this one is a bit tougher. My tactic is to approch approach approach. You can react to omega beams. If they jump, keep approaching. If they use omega beams react, and if they teleport Green Lanterns walk speed is enough to get out of the way. You can full combo punish with a b1 off of air teleport.
Ivy: Ivy and Green Lantern are pretty comprable with zoning, mainly because we can trade with her. She ha really weak wake ups, so beat her down when you get the chance. Get up close and personal. Careful of her string that hits low. It is advancing and doesn't look low unless you know. When her trait is out, you can challenge, but it is sometimes better to eat the chip if you have a lead. Watch for her f3, which goes across like half the screen. Ivy players like to distract you and then throw that out there.
Redhood: We can punish his air guns if we space well and punish with b1 combo. His wakeups are tough because they throw off a lot of our game. They are both either advancing and retreating and both do multi hits. Zoning is pretty even. This matchup reminds me of Harley, in the respect that you need to know what the opponent will try to do with their character so you can be ready to act when you can. Delay wakeup and roll wakeup are effective in countering his mine set-up game.
Hope this helped a little bit! I am not the greatest player, but have been practicing for a few months now so take what you will from it!