Just to be clear, you are talking about the Lvl 55 “Fall of the Outcasts” Timeline Boss?
All the MoF bosses are very mechnanical and invole very specific mechanics that can be easily dealt with, once you unterstood them.
Also literally all of the incoming damage is 100% avoidable.
Since it sounds, that you are very upset, i guess you don’t mind if i spoiler you he exact mechanics?
Just in case a spoiler here:
Boss Mechanics explained
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Boss is invulerable until all 4 Soul Vessels (in the 4 corners of the arena) are destroyed
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After each Soul Vessel dies, the quarter of the arena gets lethal. When all 4 are dead you need to fight the boss in the middle
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The big fire circle and lines are probably obvious to avoid
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Boss has a “slam”, where after a short delay a spike comes out of the ground and hit you, you need to keep moving to avoid that.
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He shoots out 3 “boomerang” necrotic projectiles, which return and do lethal damage at the center, where they all meet. Once he shoots them out, try and stay away from the center point, where he initally shooted them out.