Fia is in Fristad Rock at Noon, guarding her reactor. She uses the Havoc Slab, making her incredibly durable and dangerous in close combat. Her bunker is a maze of traps and Eternalists. The biggest threat is her reactor, which she'll detonate if she feels cornered. Stealth is paramount. Use a silenced weapon for a quick headshot or lure her into a trap. If with Charlie, kill her first.
