Firstly, this thread is about a completely different bug which makes it confusing. Had you searched for Shan Jitong in the forum you would’ve found this thread - Pei-back quest bugged mop
This was the suggestion there ‘I did this quest a couple of days ago on a lower level character and it was fine. I wonder if you’re killing the colossus too quickly as you’re a much higher level. Shan Jitong should be there all the time, pulling your soul from your body while you’re trying to kill the colossus and you have to get back to your body several times during the fight. When it’s dead, I think Shan says something then becomes attackable.
Basically, you need either to take your armour off or use something like the soft foam sword to kill the colossus.’
Have you tried that?