Boss

Bosses are large, difficult enemies which the player encounters in the Ruins, Weald, Warrens or the Cove.

Bosses tend to have a large health pool and have a gimmick that your party must get around.

Unlocking bosses
Short missions of the same difficulties as the boss (i.e. apprentice expeditions for apprentice boss) will increase the unlocking progress by 50%, medium by 75%, long by 100%.
 * Unlocking bosses requires successful expeditions in the relevant location. The longer the expedition, the more progress is made towards unlocking the boss.

Short missions of difficulties lower than the boss difficulty will increase the unlocking progress by a lesser degree (seems to be 33% for short apprentice missions for veteran boss?).

Excess unlock progression from expeditions which unlock the boss DO appear to carry over to the next boss. (egs. at 50% progression to unlock apprentice hag, an apprentice medium mission will unlock the boss AND progress to unlocking the apprentice Brigand Pounder by 25%)

Tips

 * When locating a boss, it is almost always the room farthest away from the entrance room. It is very rare to find a boss in a non-end room (excluding the warrens), but it does happen.