Turn the bulette into a landshark -- let it swim through the ground. You don't have to give it the ability to phase through earth to do this; many RL burrowing creatures simply wiggle or 'swim' through the soil rather than digging with claws (& in these cases tend to have highly reduced legs or none at all). Replace the claw attacks with something else, remove the legs, & let it swim through loose soil (& possibly bump the speed, too; 20' would make for a more interesting chase I think). This also brings in the rocks as refuge idea from "Tremors", at least until it rams them enough to knock the characters off or just shatter the stones. Oooh, hrm, that might be an interesting ability to add, actually; give it some sort special ram attack at a bonus to shatter stone objects (like, say, the foundation of the keep).