It's the same distribution as in WGT:
Belt & Pants = First boss (Flesh Sculptor)
Chest & Shoulders = Second Boss (Flesh Abomination)
Helm & Gloves = Final boss (Lord Warden)
Boots = All "mini" bosses (Overfiend, The Council, Watcher)
Note: You can distinguish a "boss" from a "mini boss" by the number of shackles they award. "Mini" bosses only award 2 shackles.