The Harbinger

The Harbinger is one of the possible first bosses that a player can encounter in The Dungeon. Due to its 2 Attack stat, it is considered to be the hardest of those bosses on higher difficulties, as Followers have only 2 Health.

Strategy
On higher difficulties, if your randomized non-Follower character has 1 or 2 Health, put a follower in the middle row. They will die on the first turn, and then you will need to move the other follower into the middle row. You may need to ignore the skeleton piles after the second or third turn and focus on the boss.

Taking the Blood of Illifar boon on floor 2 to get a level 2 Champion trivializes all of the floor 3 Bosses, even on the highest difficulties.