You could have another object, which is an invisible sprite and has its own collision box. This will be called CriticalHitBox.
Then you can add a few ImagePoints on your Boss sprite, and when you need to define the place of the hitbox, you can do:
Boss > Spawn another object (CriticalHitBox) on Imagepoint choose(1,2,3,...,n).
This will make a random selection between the Imagepoints you predefined.
Gah, I kinda suck at explaining...
Edit: I forgot to say that you'd have to Pin the object to the Boss, as the person above said.