Prodeus

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.

I earned this on my first try on the Memoriam level. Their are only 45 enemies in a wide open area and no enemies spawn until you hit the switch out in the middle end allowing you to max out your ammo. After hitting the switch you must quickly jump up to the armor on your left and up to the rock area beyond it, double jump is required. You can hide behind the rock pillar here and take your time, enemies will spawn slowly as you pick them off, although to get them to spawn you may have to walk out to your right along the memorial, quickly heading back behind the pillar as they spawn.

Besides the usual fireball throwing and flying enemies you'll need to watch out for a shotgunner who stays in the middle and the sniper who will appear above the starting area after the halfway point. The rest of the enemies including some shotgunners will pile up along the memorial beneath you and it's easiest to bounce grenades down there to wipe them out.

You're better off doing this achievement at the same time as A Really Big Deal as the single secret is easy and you have to kill every enemy in the level to finish it anyways. Also the other solution states you have to play it on the hardest difficulty but I didn't test that.