Finally managed it!
- Unplugged all my discs that wasn't supposed to hold the Win 8 installation
- Entered the cmd prompt at the first win 8 installation screen (choosing language etc.) through shift+F10 and then:
Select disc 0
Clean
create partition primary size=xxx (xxx=whatever you disc size is or how much size you want to use of your disc)
Exit
- I then went with advanced installation to make sure that no extra partitions had been created (system reserved). Here I also had another chance to format and resize the partition I just created.
- Completed Win 8 installation
- Plugged in my other discs
- Restored boot order in BIOS to make the Chimera disk have highest prio
- Booted OSX Mountain Lion
- Reinstalled Chimera
Voila!