Thanks for the advice everyone.
I should have mentioned that it's the allen head that is rounded not the threads. I swear I didn't make this super tight as I've done it many times before, it's weird. I think I may have put it together without putting the washer between the screw head and the frame. I think the screw and rocker are both aluminum
I can't get a vice grip on the head or cut a slot across the head as it's counter sunk. I may be able to cut a slot in the other side though. It threads in to a nut and some of the screw sticks out.
I'll try the next size allen key next and then the jb weld but is there a way I can release the seize first? WD40?