We played several games. I'll write down some additional things I noticed. Each specialist can use all of their abilities; they always go down when their HP reaches 0. Factors that cause death are exsanguination or if you are taken down. If you die during a wave, you will not be able to go out again unless you finish that wave with your remaining allies. When you reenter the battle, your equipment returns to its initial state. There are several types of Geist, and each wave seems to have a specific type of Geist that may appear. They generally have more enhanced HP in the later waves and may have ranged weapons such as smoke grenades, incendiary grenades, shotguns and LMGs.
The equipment available for purchase in each phase of the crate is as follows.
Phase 1: None
Phase 2: DM7, MCS-880, armor plates
Phase 3: AM40, BFP50
Phase 4: C5, T-UGS, incendiary grenades
Phase 5: PP-29
Phase 6: VHX-D3, ammunition boxes
Phase 7: PKP
Phase 8: Identical
The optimal squads in my opinion are Farook, Angel, Boris, and Crawford. Farook is a valuable resource in this game mode because there is no means of recovery and he is quick to revive. Angel is in principle not able to replenish ammunition until he gets the ammunition crate in Phase 6 (ammunition is maxed out when re-purchased in the crate), so he does not have to worry about ammunition when he is in the squad. Boris can quickly respond to a wide range of enemies by setting up a sentry in an open area, staying there himself, and activating his passive. Crawford's gatlings are extremely effective because of the fixed locations where enemies appear, and he never has to worry about ammunition. Conversely, Macay, Casper, Rao, and Blasco will not be able to fully utilize their capabilities.
The ideal winning pattern, in my opinion, is to get the T-UGS, ammo boxes, and incendiary grenades and PKP for everyone, shared with squad members, as soon as possible. Then figure out when the Geist with ranged weapons will appear. This Geist has extremely high HP and good shooting accuracy, so even if you are not on your guard, you are at risk of going down. Also, you need to occupy the area to advance the phase. You can set up in front of the printer's eye, but you cannot proceed unless someone occupies the area. Find the best fighting position in each wave.