The Collector

The Collector is an Eldritch/Human miniboss who has a 5% chance to spawn if your inventory is over 65% full. Drops pazzling trapezohedron, a gem worth 2500 gold, or one of three unique trinkets: Barristan's Head, Dismas's Head, or Junia's Head.

Strategy
Collector doesnt have the highest of stun resistances or speed, you should have a chance to stun him on first round before he uses Collect Call and damage him significantly.

Once Collect Call is used, target Collected Highwayman first, as he deals high damage and bleeds you. Stun Collected Man-at-Arms to break his guard on Collector or Highwayman, ignore Collected Vestal. The ideal composition to deal with is 2 Vestals and Collector, Vestals dont have offensive abilities, you can outdamage their heals and sometimes they will waste their turn on buffing, from rank 3 Collector usually uses his weakest move, Show Collection.

After the fight you can spend a few turns healing, Collected Vestals and Man-at-Arms are harmless.