There are many cheese spots, just YouTube it.

It sucks I won’t be doing it again. Use osteo and disorienting grenade launcher plus machine gun strand. Strand machine gun rips down his shield and disorienting launcher helps control all adds. Once you get to final it’s easier just run and shoot osteo until finished.

I felt the same frustration. I know it scales difficulty for more people, but I think for every boss fight in the Lightfall campaign I had to post a LFG to get through... except this one, it was way too hard with multiple people dying to where I had to cheese it by hiding under stairs and peppering the tormentors and Calus with a primary (as posted in the link below). Felt super unrewarding. Witch Queen campaign there were times I needed help from others but did a lot solo, it was more about setting my armor right for different incoming burns, but this was just ridiculous. I played weekly missions previously for pinnacle rewards, I have no desire to go back into the Lightfall missions even if for a pinnacle.

I burned the first phase with LFR from a catwalk and used strand to grapple to safety if things got too hot. 2nd phase I just covered him in witherhoard and ran in circles until he died from DoT 😂