I think there's depth here that you guys have missed because you just entered solo or didn't get very far.
You're given an hour time limit because you're supposed to progressively push their defenses (Gates), hence the name Sortie. You unlock gates by finding keys in chests. You unlock fighting NMs by finding device items in chests.
Using the temporary items to activate devices, you can fight Vagary-style bosses in special unmapped areas.
There's a Gate #D which is accessible through multiple passages (A,B,C confirmed, but I assume all).
I'm assuming that Gate #D is a mega boss area.
Here's a few tricks I -picked up from my first run.
- Treat it like Vagary. Have a paladin pull EVERYTHING and use AoE to cleave it down. Our AoE damage wasn't nerfed. You can also use Fell Cleave, Sonic Thrust, etc. If you use Physical AoE, make sure you use capped DT because it pulls everything onto you and your healer should be ready.
- The further you get in, the more chests you get, meaning the more NMs you get.
- You can get "Plates" out of chests. These plates disable potent abilities on the NMs.
- The NMs we fought had a bunch of knockback, silencega, and other really nasty things. More plates probably mean these are alleviated.