im just gonna come out and say from experience which matches i believe manta has the advantage in so far.
vs Brainiac: 6-4
brainiac's air game doesnt exist in this mu due to missiles, which means he's gotta approach from the ground, manta controls the space better and faster than he does in this MU, and if manta uses beams and missiles on oki brainiac can't really wake up either.
vs Flash: 6-4
completely out ranges the flash and controls the space he wants to get into very well, flash is going to spend the majority of the match trying to get in, and even when he does, the range on manta's d1 can cause problems even then.
vs Darkseid: 6-4 (potentially worse?)
Darkseid doesnt really excel anywhere on the screen in this MU. He can't really zone due to manta's teleport and the recovery time on his beams, darkseid can't really do much in the neutral, and if darkseid whiffs an omega beam from closer than full screen, theres a good chance manta's going to punish with with his charge or get in or punish with f21. Also, manta very easily anti-airs darkseid, having plenty of options to do so including a good d2. also, darkseid is the only character in the game that mb teleport will not whiff on if blocked.