Uhhh. Since Bishop absorbs energy including kinetic im shur he would absorb a Namor punch. However im not sure if Bishop has ever been punched by someone as strong as Subby.
Bishop does have some pretty good weapons though. But i doubt hed be able to land a hit.
Anyhow once Namor sees that Bishop absorbs the punches(if he can), he'll resort to grabbing him and breaking his neck.
Well Namor would lose his kinetic energy he gives out to Bishop would not do anything and Namor wich he hardly uses also can shoot electricity out of his hands wich he can not use.
Other energy types go too. He once absorbed an radioactive energy blast from the Presence (The Russian guy). Presence had sent Hulk flying earlier with similar blast.
As for the fight, Namor will tear his head away and give it to Atlantean children to play with.
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Bishop is completely dependent on two things. One on having energy reserves. Well, three things. Two on his guns. and three on futuristic tech.
Namor would have to survive long enough for Bishop to only have his guns left in order to beat him.
As an energy absorber, Bishops ability to channel and trans-mutate energy would probably be enough to destroy most hero's.
Bishop is however vulnerable to all forms of physical attacks. He only has the strength and invul. of a normal man in peak condition.
He can't absorb the physical, only energy derived from an energy "beam".