Uber Elder down.
I'm ashamed to say it took me many many deaths to get it down, I think I was on my 6th set or so, which is pretty awful.
I had to rework my char quite a lot, I had a ton of damage to spare, but I kept getting killed. I respeced into spectre life ascendancy so they wouldn't die(even then a couple died every now and then, damage is hardcore), and I respeced out of Necromantic Aegis+Victario frenzy/power charge on minions to get Unwavering Stance(since I kept getting stunned and then hit by balls or beam or slam or whatever else) and more health(with a Lioneye's Remorse as my shield, bumping me from about 7k hp 1k ES to 8.2k hp 900 ES). I also got used to the fight a bit more, but there was still some bullshit and the set just before I cleared I wiped so close, but Elder fucking slammed my ass out, didn't even know he had a slam, I thought only Shaper did. Also wasted a set because I bought a tempest prophecy for the challenge to take a break, then went to eat, forgot about it, reopen uber elder, and I had fire tempest in that shit, which made it way harder for no reason.
Had to sell a bunch of shit to buy the sets, mostly maps and an exalt(although I still have over an exalt worth of chaos so selling the exalt wasn't necessary). Didn't want to buy maps since I didn't want it to despawn, so had to buy fragments directly instead.
In comparison to every other fight difficulty wasn't even close though, there's A LOT more shit going on, and A LOT more damage, and you need to fight for A LOT longer due to phase switches and invuln phases and shit like that. I could push into the first summon phase in like 10secs(25% on both shaper and elder) but after that with all the delaying and running around and shit it made it so I couldn't really skip mechanics like I could with other bosses(for example normal shaper I barely saw anything because he was just getting pushed into next phases in seconds). It's also kind of a fucked fight if you can't leech/don't have super high regen. I usually rely on potions but it was rough, had to buy a Blood of the Karui for efficiency since healing 8k health with seething flasks isn't great and the minor god thing was pretty useful to get some free flask charges back.
Anyway with this done, I basically beat PoE. Cleared every fight(well I guess not Uber Atziri but I don't want to bother it looks boring and it wouldn't be much of a challenge with how much damage this build does) and every map(not all uniques I guess, I should still do halls of grandmaster since iirc that's hard).