If you want high scoring bosses you need to friend the high level players (>200).
I check the battle list of high bosses, and then use this (or the ranking list) to send friend requests.
The way it works is if they already have too many friends, it gives an error message. Move on to the next. You will also need to complete missions to level,and regularly "prune" your list of friends if they are too much lower than you. Don't ditch everyone though - low level players send damaged foes you can kill with one hit, and earn abattle cube for finishing off
On the lookout for MISB Headmaster Highbrow, Takara or Hasbro. I'm sure I could make you a sweet deal!