Ultra Hard has essential difference that goes beyond damage multipliers; Your elemental powers and Tear are weakened below half they are on other difficulties.
Enemies detection range and sensitivity goes to a point that's almost ridiculous; keeping a Stealth outfit is no longer an advantage, its a must-have. Whistling twice too soon, accidentally hitting foes with rock or even having enemies stand too close to the tall grass you're hiding risk immediate detection. Ammunition that causes any explosion will guaranteed expose you unless your Stealth is above a certain threshold.
The combat is extremely dangerous: Not only Aloy is very fragile, even after level 40 with Shield Weaver, the enemy Ai and combat behavior changes. Machines become hyper-aggresive and will not quee attacks, they also do not flee combat unless they lost sight of you. The Shield Weaver's shield could go off by just one body Slam from a normal Watcher. The human become very tough, some can even survive headshots from modified Hardpoints.
My only advice is you gotta play like Aloy would, watch every movement, slip through every possible blind spot, exploit all possible weakness and count your arrows.