Sounds like you skipped an area or two, currently Plains are the highest difficulty areas:

1. Meadows - Eikthyr
2. Black Forest - The Elder
3. Swamp - Bonemass
4. Mountains - Moder
5. Plains - Yagluth

Oceans and Mistlands, too, but they're pretty bare at the moment and neither have their respective boss yet.

If you've already beaten Bonemass, then head into the mountains, gear up with frost resist, plenty of the best foods you can make, and the wishbone. You'll probably be there awhile, so get comfy.