How to kill Pinnacle bosses?

I'm currently running an attribute stacker gemling legionnaire using storm wave with upheaval; lvl 89, got nearly 350 str, 225+ dex, and nearly 350 int, maxed resistances, 3.7k health, 500+ es, PoCG, and HoWA. I can run juiced t15s while dying fairly infrequently, but I can't even beat Xesht without help. How do people get the damage/resilience they need to take on Pinnacle bosses?

Pinnacle bosses generally have higher resists / armour than most enemies, think some go up to 70%. In order for me to get the dmg to fight Xesht I have to swap a few things in my build that I wouldnt use in maps.

I'm dealing cold damage mostly, so I make sure i have:
-A way to apply cold exposure
-Hypothermia (cursed ground is handy)
-More cold penetration
-Shock (with increased magnitude if possible)

Some other things I've found useful are freeze, stuns and stacking slows on the boss. Slows have reduced effect on bosses, but stacking maim, chill and hinder still makes a huge difference and lets me spend less time dodging, more time hitting.(can make their voicelines no longer line up with the attack animation though).