JUST FINISHED HZD ON ULTRA HARD
and holy shit that endfight was TOUGH! When I started playing the story I checked multiple times whether I had actually chosen ultra hard because thanks to New Game + I already had all the weapons and the highest Level, so it was pretty easy most of the time. I did die here and there along the way, but that was usually due to not paying that much attention to the healthbar (because the machines weren't that much harder to kill, I kinda forgot that they were much stronger so getting hit by them was much worse for Aloy) and one time because I'd forgotten that I wasn't doing the cauldrons and therefore obviously couldn't override all the machines xD.
I'd stocked up on ammo and some healing items but was still woefully underprepared for the final fight on my first try. Maybe it's simply because both the fight against Helis and the one on the Spire happen in such cramped spaces and stealth is absolutely no option. Died so many times in both parts and even got a Game Over for running out of time on the Spire once. I didn't have a problem with the middle bit where you repeatedly shoot Petra's experimental weapon at waves of approaching machines though.
Anyway so after I got checkpointed on the Spire without any health items left and getting more and more frustrated I aborted the mission and spent a bunch more hours hunting boars and other game. Also bought some Adept weapons (only to find out that the Adept Sharpshot Bow isn't as good as the Banuk Powershot, so I didn't even need it - didn't want to go to the Cut to get the Adept Powershot though) which helped a bit. The Adept Blast Sling did considerably more damage than the normal one, the Adept Tripcaster is great in theory but I never got the time to use it in the heat of battle.
On my second try the bit with Helis felt even tougher than the first time around, but after many tries I made it through again. The bit after that felt harder than on my first try as well, but I did make it without dying. And then the Spire. Died again and again and again because of all the chaos, felt like I came pretty close a couple of times but died again. Then FINALLY managed to get the Deathbringer's health down enough to explode one of its guns and got excited in a sort of pessimistic way xD - at that point I had a small handful of the smaller healing potions left and a tiny amount of herbs - I did my best to avoid the attacks, managed to actually chug some small potions when necessary and then blew up the Deathbringer's other gun. There were 46 seconds left on the clock, my heart was racing and my hands were shaking slightly. I only had like two herbs worth of healing power left, but I managed to chuck a few more bombs at the Deathbringer and it collapsed. Didn't even believe it at first because I must have tried to get to that point about 30 or 40 times by then.
Anyway, great game, would recommend, but if - like me - you're more in it for the story, I'd say play on normal or even easy difficulty, because ultra hard mode may be bad for your blood pressure XD