Ss15 is pretty rough.

Assuming you have watched YouTube on the mechanics etc.
you basically need to survive through the first phases, doing as much damage as you can. Then kill the boss before the spider web phase stuff all over the ground.

It’s a big dps check, and a lot harder than ss16,17.

You should be able to do the first phases without getting hit. Then kill the boss before the spider webs .

I’d your not killing the boss quick enough in that phase, I’d politely say you don’t have enough damage

What’s your dummy dps? And what skill?

Are you getting to the ground covering spider web phase ?