So the achievement description for this on Xbox and the Xbox app on Windows is wrong. In game however the description states that you need to "Finish a Level Without Dying On Hardest
Difficulty" in this case is ULTRA HARD.
This can be done on the first level, Sacrum. Enemies overall are a little tougher and take a couple shots more to kill. The blue glowing demons right at the start throw fireballs way faster than before, always strafe.
If you aren't going for
Run past walkers to finish the level faster, they don't jump/climb so they can easily be avoided.