Holly's Request is a four-part side quest that becomes available after you complete the main story quest "Slaying the Hero Holly" and restore her bond. The quest tasks you with helping Holly test a new Jail-enhancement device by defeating enemies in specific locations across the Undead Forest. Completing all four requests grants you new skills, booster slots, and memories that expand Holly's character story.
Quick answer: Complete four separate Bride of the Immortal boss fights at the Snowy Mountain Amusement Park, Housing Annex, Abandoned Hilltop Church, and Mountain Hamlet to finish Holly's Request.

Unlock Requirements
Holly's Request becomes available only after you finish "Slaying the Hero Holly," the main story quest where you defeat Holly at her cocoon in the Sanatorium. Once that quest is complete, return to MagMell in the past timeline and speak with Holly. You'll find her upstairs from Valentin's office. She'll explain that she needs your help testing her new device and ask you to meet her at the Snowy Mountain Amusement Park.
Request 1: Snowy Mountain Amusement Park
Step 1: Travel to the Snowy Mountain Amusement Park Mistle in the Undead Forest with Holly as your partner. Proceed through the area until you reach a boss door at the end of the trail.

Step 2: Defeat the Bride of the Immortal. This boss uses a variety of stone-based attacks and projectiles. Block or dodge the Energy Barrage (multiple orbs that fire downward), and watch for the Stone Energy Blast, which fires a single line of purple energy that builds up the stone status effect on your character. The Stone Energy Wave attack sends out a wave in front of the boss—dodge to the side or block to avoid it and deal damage while she's stuck in the animation. She also performs forward slashes (parryable if you're quick) and can teleport to perform multiple slashes or a stab.
After defeating the boss, a cutscene plays. You receive an additional Booster Slot and the Cognitive Zeal skill.

Request 2: Housing Annex
Step 1: Travel to the Forest Complex Mistle and head east to the Housing Annex with Holly as your partner.
Step 2: Enter the boss door and defeat another Bride of the Immortal. This fight uses the same moveset as the first encounter, so apply the same strategies: dodge or block the stone attacks, parry the slashes when possible, and capitalize on openings after her wave attack.
Upon victory, you earn the Blood Sacrifice skill.

Request 3: Abandoned Hilltop Church
Step 1: Go to the Abandoned Hilltop Church Mistle, north of the Housing Annex. Enter the dungeon and follow the path on the right until you reach the boss area.
Step 2: Defeat the third Bride of the Immortal. The boss uses the same attacks as before. This encounter rewards you with a Forma Extension after victory.
Request 4: Mountain Hamlet
Step 1: Travel to the Mountain Hamlet Mistle in the southwestern portion of the Undead Forest. Enter the dungeon and follow the wall on your right to locate the boss arena.
Step 2: Defeat the final Bride of the Immortal. Use the same tactics as the previous three fights. After this victory, you receive another Booster Slot and the Ice Spikes skill, completing Holly's Request.

Rewards and Completion
Finishing all four requests grants you two additional Booster Slots (one from Request 1 and one from Request 4), plus three new skills: Cognitive Zeal, Blood Sacrifice, Forma Extension, and Ice Spikes. Each fight also includes a memory sequence that provides story context for Holly's character. Once you complete the final request, the quest closes and you can move on to other side quests or story content.