The First Location is really easy because it's a Open Field and you can't really miss it

The Second Location is a little bit hard to find because there is not a Special point where he is Spawning.
I found him in the location on the Picture if you can't find him on the same Location just search around the Area you are gonna find him don't worry.

The Third Location is Really Easy you just need to go southwest to the Inverted Pyramide on top of it you can Fight the Boss.

The fourth Boss is really easy to find too because you just need to get behind the Giant tower and you can find him just on the Ground in the Wasteland.

Before you can Fight the correct Boss for the Hexe quest you need to kill the Dragon to enter the Otherworld.

After you Killed the Dragon the Portal appeared and you can Enter the Otherworld.

If you enter the Portal just go straight and the Boss appeared

But if you fight the Boss watch out he is only lvl 600 but he is really strong and can kill some of you allies with only 1 hit and the Area to fight him is really small

Good Luck