In my campaign, the PCs have been tasked with removing a group of bulettes from a mining claim. I plan on using standard bulettes as a lead-up to a 'boss fight' where I would like a larger (perhaps cr 11 or 12) bulette for the group to kill. How might I go about advancing him beyond simply advancing beast HD?

If the answer is simply 'advance beast HD and increase size', then how many beast HD is appropriate to make a CR 11 or 12?