Hello everyone, to keep the story as short as possible, I've got the same bug reported by many in previous posts. Trouble is that I've tried almost everything that was written around here as a possible solution and still I've miserably failed.
Suggestions:
1) punch the dude towards the roof barrier on the side with the sea - tried it for a couple of hrs but every time there is one punch that throws him towards the roof and I can't even get his health in half before that happens; if I stop even for one split second from punching to align myself properly so that I can keep him between me and the barrier with my punches and he's not off balance he's gone for good;
2) lure him on the roof or even to street level and beat him up there - tried this a couple of times and managed to get him on the street; trouble is that as soon as his sign becomes red he dissapears; also punching him on the roof or in the street makes him dissapear immediately;
3)come from behind him so that he cannot see you and throw a knife and then beat him up - this was closest to success I've ever got, with the knife I've reduced his health to about half but then no matter if I punch him on the platform or if he's thrown towards the roof he dissapears in about 2 seconds after being hit by the throwing knife;
4)grab him or punch him so that he doesn't cross the center line to the platform - as soon as he's grabbed he's gone; if he sees me face to face even my first punch will not hit him, he'd be gone by then;
5)do not use the venetian cape in this mission - no effect;
6)lower graphic settings to minimum - no effect.

Any of you guys tried something different that worked?