Horizon Zero Dawn - Ultra Hard No Upgrade Run - Final Boss Corrupted Deathbringer Part 3
Unlike parts 1 and 2, this is where things become extremely problematic. I had previously thought this boss had a checkpoint every 25% health, but it turns out it's only at 75% and 50%, meaning you have to do the last half of his health bar in one go. Under Ultra Hard NUR conditions... this is frustratingly barely within the realm of possibility.
The main issue is the second wave of machines here. It contains a Bellowback and a Corruptor that are extremely fond of locking on to Aloy and following her around. Both of them have long range attacks, so it's not enough to merely lose them, you have to escape them by a substantial distance. If your NPC allies aren't being cooperative, this can eat up over a minute of the fight.
After seeing the timer run out nearly 20 times (having never previously seen that happen), I finally settled on using the Deathbringer's own gun. It takes 45 to 50 shots on either the far right or far left turret to knock them off and make a Deathbringer Gun. It comes with 60 ammo and does 70 damage a hit to a weak point. If you can pull this off and land even most of the bullets, this is a dramatic surge of damage compared to your measly bow.
Shooting the gun off isn't a big deal, but finding an opportunity for the Deathbringer to let you use it--and getting at least most of the bullets out--before the time limit runs out, is a serious ordeal. You'll notice in the video here I'm basically popping potions constantly to try to survive the incoming attacks so I can get my damage out.
I think this strategy could be improved upon. There are two potential Deathbringer Guns, one on each side. I believe there is plenty of time within the battle to shoot each one off. You would still have the problem with finding an opportunity to pick them both up and use them, but if you could manage that, I think the two guns would kill the boss without any further DPS needs from Aloy.
I know that this fight looks extremely dramatic with me ending with only 8 seconds left to go, but don't take that as too daunting. This is a fairly unlucky run as far as things go. The corrupted machines harass me a decent bit, and I miss way too much with the Deathbringer Gun when I finally get my hands on it. On my best run using this strategy, I had over 90 seconds remaining, I just forgot to do my stats check so I threw that video away.