Thanks you, Im really glad you like it!
For the HP I made a small calculation, taking into account the playercount and level, resulting to their average damage output, the targeted combat length, the monsters low AC and therefore the chance of hitting the creature and some time for the party to figure the situation out. That resulted in their HP. I compared it with some of Kelsey's work and tested it. For me it seems fitting for this kind of boss but you are free to adjust it to your liking.