Then it's broken in the patch, because I've faced it 4 times now, done plenty of damage to it, with only short rests, and by the time it comes back it has full health again.. As to your solution, gaming a broken system being the most effective way to beat an enemy, means that the encounter and/or enemy design is broken, and it's the complete antithesis of DnD.
I've been investigating a bug where someone claims that they killed multiple bulettes, because I want to collect them, so I've killed one at least 15 times this patch. Most of those times letting it burrow on purpose. It regains ~20 hp when it burrows in and another ~20 hp when it burrows out, if you don't rest.
That being said, what I will concede, is that, if you are invisible or out of range when it comes up after being burrowed, it will auto heal for full health within a couple seconds.
Edit:Also, in your case, I'm not sure if you can even have 5 shots at it. I believe it only triggers in 4 locations, and they seem to be one time use.
