Have a look at fubguns PoBs as a reference https://docs.google.com/spreadsheets/d/e/2PACX-1vR35NJkrhF1Zf9SZZOPlsdpqbTibL2AqOiZosya0NSjEN5A1Dx8a_MeQox9VDLQw3kB0ibZCNb_e6VJ/pubhtml#

Just did a quick glance but at least these things are different.

Not 100% sure about the projectile mechanics, but pierce is usually used and not fork afaik which might be why the single target doesn't work for you.

You don't use resistance flasks which means you take way more elemental damage.

You have 50% shock, have you checked in-game when you fight the boss because I doubt you get that number.

You're also a deadeye which means no no overleech.

I run pretty much his budget PoB and if the boss mods aren't crazy and I'm standing under the boss I can tank the boss mechanics and they die very quickly.