Battle Brothers

Geists are blue ghost type enemies that are found accompanying the undead. They can be found in wandering parties, during contracts, and in dungeon locations (usually Fallen Fortress locations, in my experience).

They have 2 types of actions, an AoE scream that damages your morale and a melee attack that ignores armor which I've only ever seen them use when a mercenary is running out of a Geist's zone of control. In battle, they will typically move starting in the 2nd round and will try to place themselves behind Weidergangers who clog up the battlefield.

As far as killing one, any hit will destroy them. Don't bother with ranged as any ranged attack has a 5% chance to hit (not to say I've never gotten one this way, but it's rare, obviously). Instead, focus on pushing your team into melee range and keep your Bannerman close so your morale stays as high as possible and he can blow his horn to rally everyone if needed.

Once you are in melee range, it's usually pretty simple. One hit, one kill.

Good luck and let me know if I need to add details!