What do you mean by the "Boss" ghosts? Which ones are those counted as? And by 90% you mean not only do I have to suck up 90 of their 100 health in one pull, but I have to finish with 90% of Luigi's health?
--- Ooh, look how disruptive I am! ~~~ ~*~ -~- ووو +++ !!!
What I mean is you fight a "Boss Ghost," it's where you try to capture the ghost in a different area. Boss ghosts: Chauncey, (the baby), Bogmire, (the purple graveyard ghost), Boolosssus, (a big Boo composed of 15 smaller Boos), and King Boo, (the Boo that has Bowser on his side). Normal Portait Ghosts (where you find in various rooms), deplete 90% in 1 suck. Bosses, however, defeat the boss ghost by having at least 90 HP (Luigi's health). Hope this is clear now.