This mission happen on Zephyria's main fortress, but different from the usual (always failure) assault mission here. An entirely different map with different number of enemies.
Condition: Enable after event 2nd Spaceship Arrives AND terraform reaching sufficient level (80+) to allow for it. There's report of needing an event linked to Original Inhabitants of Mars research, but no repeated confirmation, most likely a bugged trigger. No need to destroy Primary Key Gate, or even other gates.
This mission is on Zephyria's main fortress icon, with first stage is Reach Location. 2nd stage is kill them all type.
First stage: A very few number of hostiles left, assumed to be the survivors of the genocidal virus we released into Martian atmosphere. There's a line of mines blocking the way pass the destroyed main gate.
Second stage: A kill them all type with some number of hostiles left but judging to be lesser than an usual mission with maxed number. The Beastmen corpses on the floors are considered to be obstacles and can block shots.
These two maps are unique, but the usual application of enemies placement left much to be desired. Even with story element of genocidal virus, there's no need to reduce enemies to that low amount.