Xenology

Xenology is one of the friendliest badges to unlock: you only need to defeat the Alien Mothership for the first time. With a 1-star difficulty and a small 2-diamond reward, this is a one-and-done milestone rather than a grind, and it pairs naturally with the late-game UFO event.

The Mothership appears during the rare alien-invasion night, so the real prerequisite is simply surviving long enough to trigger it and being ready when it descends. Stock up on your strongest weapons and a healthy supply of ammo or melee durability beforehand, because once the encounter starts you will be fending off smaller alien units while chipping away at the main ship. Keep moving to avoid its attacks and focus fire whenever its weak point is exposed.

Since you only need a single victory, the best plan is to tackle it with a coordinated team. Group damage drops the Mothership far faster than soloing, and having teammates to revive you removes most of the risk. Save it for a run where your gear is fully upgraded, commit to the fight, and the 2 diamonds plus the badge are yours on the first clear.