For me it was the plains boss because it took me quite a bit of time exploring before I found his location, then just the shear difficulty jump from Mountains to Plains is quite a difference with having to be on constant guard of you food, raiding Fulling villages to find the Barley, and flax, then making a safe place in the plains to grow both on a farm. Getting my gear fully upgraded didn't take long, and the boss in general isn't to bad if you use silver arrows, and sword since he's undead. Its the plains biome itself that I think was the true boss in the progression after The Mountains.