My advice is to limit how often it can use it's breath. I found that it activates instantly, too often (sometimes every turn), and provides the boss with an extra turn. It should only be used twice, once at half health remaining, and once at 1/4 remaining. I have been wasted by two turns where it spammed breath and tail swipe at the same time, every time.
Viewing post in Risk of Infection v3.1 patch comments
It is too powerful for a one turn cooldown. maybe if it didn't cause ignite, but as it is, it needs a 3 turn cooldown to make it remotely viable. Also, get rid of the free turn it provides, that is functionally making it like the boss gets two turns in one move. No matter how difficult you want a boss to be, they should follow similar rules to the player to give it a fair chance.