Once he gets past the traps/technology and stuff,
Namor would beat Manta like a rented mule.
A gang-land beat down would look like a walk through a park
compared to pimped-up back-handing Namor gives Manta.
The last sound Black Manta hears before drifting into cozy blackness......
IMPERIOUS REX BEEYATCH.
pimp
Yes, for this battle to work we have to forget the fact that in comic books the villains nearly never win.
I am thinking back to some really old Aquaman comics I read about where he has Aquaman's son trapped in an air bubble and forces him to choose between his sidekick and son. As far as villains go, Black Manta was shown as being one of the most cold-hearted back in the day.
(Besides, he has the robot voice.)