By picking a Single Boss target at any setting you will get the name of the boss you need to kill in the adventure window. Just attribute the name to the layout to the rare and cycle through the adventures until you get the one you need.

-The Forgotten Wastes: Towering Monstrosity, Beguiler Chetrea, The Silent Choir
-The Frosted Halls: Ianlai the Corrupted, Balaeluo the Inflictor, The Dream Defiler.
-The Grand Library: Bilach of Avarice, Riddlesting, Sionul the Savage
-The Heart of the Menagerie: Synarcanic Essence, Synarcanic Presence, The Fireborn
-The Hushed Banquet: The Prime Steward, Charaex the Mastodon, Ancient Marrow
-The Maw of the Menagerie: Shyar the Shard, Glacius, Grand Sage Havori
-The Morbid Laboratory: The Flesh of Osoros, Painsinger, The Nibbler
-The Silent Gallery: Zubruiz the Mindful, Vkaziz the Defiler, Soulstorm
-The Spider Den: Elder Phalanea, Pulgrish the Ancient, Summoner Sghrene
-The Theater of Imprisoned Horrors: Orgogzolok, Soulroar, Jebreusulun

With the important caveat that, when requesting a dungeon, any of the last 5 dungeons that you have started will not be included in the possible dungeons you request next.

Goodurden!

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