Ok, at wit's end and a little frustrated at this point... after I chop off 3 tentacles, the ruins fall on the Karyan and trap it. How the f*** do I destroy the last tentacle? If I leave cover for even a second I get pelted to death with rocks. Yes, I know about the QT event and yes I'm doing it successfully... what am I missing here?

Ok, ~20 reloads and much frustration later, figured it out. You just need to climb onto the stone tower that falls onto the Karyan after the 3rd tentacle is destroyed. The rest is a cinematic.

thanks Pal
this will come in handy:)