Why do they keep Atziri harder than delve 1000??

I can only assume you play melee physical build.

Atziri is one of the fun encounters, and i'm happy it's part of the content.

there's a trick.

atziri has phases:

1. when she's vulnerable, just attacking you.
2. when she's inside her shield, summoning allies.
3. when she spawns 4 identical replicas of her. i assume this is your problem.
each replica holds a different item. one of them holds a mirror. this will reflect you your physical damage. go to other direction and kill other replica.
once you get that, that fight is easier than normal T8 boss.