Big boss monsters are almost always better served by giving them some extra company than they are more HP. More defenses/DR/magic resistance can help but it mostly just frustrates PCs.
If you really want a classic boss fight against a single creature, the
first place to start is 6x HP and either area attacks that are large enough to hit most of the party, or some other way of duplicating the attack output of a group of Worthy monsters (four Extra Attacks and Long Arms works). Another thing that's pretty important is a way to prevent Resisted spells from ending the fight with a single die roll - if it's a large monster, then the scaling of FP costs based on SM helps here, but may be a bit binary.
There's a whole other thread here on Boss Monster Design that doesn't have anything to do with the adventure so I'll take this elsewhere.
EDIT:
new thread here