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?
You have several options to advance monsters.
For a bulette, adding class levels doesn't make a lot of sense so we'll leave that one aside.
Increasing HD is more than adequately covered (including the resulting CR) under the Monster Advancement rules available in your Monster Manual or on the SRD.
You may also choose to apply a wide variety of templates, all of which clearly specify their resulting CR adjustment.
Specifically for the bulette, go look at its SRD page and there's an Advanced Bulette variant that shows the effect of slapping the simple advanced template on it.
However, all this advancement tends to break down if you're amping up CRs over the base by too large of a margin - in this case you're going to end up with a real high attack/damage bulette that your PCs will immediately have as a pet because of its pathetic Will save. I'd use the demonic Xenarth variation instead, advancing it some.
You could give it more hit points and AC, but that will simply make the fight longer, not more interesting. It might even make it less interesting for the players. What you should do is make the bigger landshark unique.
AngryGM has written a series of articles on boss monsters. His take on the idea is that boss monsters have several distinct pools of HP and as the party deplete pools, the monster's behaviour changes.
For example, your boss bulette could have three distinct phases. The first phase is a big, heavily-armoured, slow-moving beast that takes a lot of punishment. When the PCs deplete it's first pool of HP, its armour has been knocked off. Now it loses its high AC but becomes an insanely fast, manic beast with lots of movement and attacks. When the PCs deplete its second pool, their wounds slow the beast down, but expose its innards, which happen to be acidic. Now it is a normally-moving landshark but is doing AoE acid damage to melee attackers.
Each phase of the beast should allow different members of the party some time in the spotlight.
I couldn't find the earlier articles on his new site, so here are the links to the 5E articles: Part 1, part 2.
Warning - AngryGM's style is blunt, in-your-face, and full of #%!@ing bad language.
Another alternative (as mentioned in another answer here) is to apply one or more templates to the bulette. The OP mentions in a comment applying the Giant and Half-Dragon templates. To me, that certainly ticks the boxes for a unique and interesting battle!
