It's also taken me forever to get Baki's fifth end but I GOT IT and I'd love to help if you like
spoiler spoiler
So 1.don't let him carry you
2.doubt him
3.check the sound
4.say you want to leave
5.refuse
6.open your eyes EACH time (I don't know if the first 2 times are necessary but maybe they are)
7.choose to FIGHT and LOSE the timed part
and there ya go♫