Playing Dormammu, I can tell you that one weakness I have with him is that he lacks good punishes, as most of his moveset has terrible startup, and so if you have a quick character with quick normals, which your character list seems to have, hi-low pressure can eat me alive because all I can do to counter someone with quick normals is advance guard. My crouching low, where he snaps his fingers for a small spark, is his quickest attack, and it’s still not nearly fast enough to break a good blockstring. Dormammu’s strength isn’t a rushdown, though someone people play him well aggressively, but mixup is his true strength. He’s not too bad until he has meter to burn, but his blue and red hand specials give him good options, so be careful to approach him when he has hands charged. Finally, if he corners you, his teleport behind you can be used for great mixup, so putting your back against a corner can put you in harm’s way.
If you can, move in on him quick, and mixup. He’s got slow normals and a huge hitbox, you keep him guessing and pressured long enough, they’ll screw up.
Really though, understand your character’s strengths and weaknesses, as well as their assists, and build your offense appropriately. Every character will contend with a threat differently, and the more you understand your team’s capabilities, the better you are equipped to counter certain playstyles. Don’t forget that Advance Guard is your friend.