It's time to tone back the map difficulty

Link your poe ninja

What bosses in particular?

Yeah game was fun in the beginning. Now you make a build and one shot everything easily. Pretty boring design

In Diablo 1, when you died, you dropped all your gear in the dungeon. It was the most fun I ever had in a game. You had to go get your 2nd best and if Butcher was involved, even 3rd best gear. If you had other people with you, they could steal your stuff. Then people complained and ruined it. A perfect blend between regular and hard-core.

Problem currently is.

Run 2-3 of the meta build and face roll

Or

Run a non meta build and struggle and get one shot.

While it's getting better , you are kinda forced into the meta build who can avoid one shot by clearing screen or 1 shot boss.

6 portals is the answer

Well, the vast majority of boss mechanics are designed to be avoided in one way or another. So, unless you rolled some insane bonus action speed mod on your map, there hardly ever should be a mechanic that just outright kills you if you look out for it.

Other than that, with how absurdly broken player power is atm, enemies (or at least bosses) actually do need to hit that hard, even though i don't agree that either should be the case. The faster and stronger the player becomes, the faster and stronger enemies need to become.

That aside, wdym with "max res"? Do you mean 75% or actually 90%? Because just 6k ES and 75% res should get you some ways, but it's not, and shouldn't be, enough to run every t15. Look into other defensive layers.
One of my characters also has ~6k ES and 75% res, but hardly ever dies (only when i do obviously reckless stuff) in maps because i also invested into curses which act as another defensive layer.
Basic t15's in PoE 2 are pretty damn easy, and only really become a challenge if you deliberate invest into boss difficulty through the atlas passive tree.

I agree that there are some bullshit oneshot mechanics. But man... 6k es is not that much for juiced high end mapping

Most one shots by bosses are heavily telegraphed (some exceptions exist aka arbiter, which needs tweaking) and the only reason you might not see them is because your build fills the screen with effects and blocks your view. Can you give an example of a boss ability you keep dying to?