@Ubiq:
Yeah, the higher the difficulty, the better the drop rate. I usually just do fifteen myself but do the thirty at least once for the sake of completion.
Usually I have a lot of trouble getting the secret bosses to appear; the only two I had much luck with are the Master of the Near Sea in Alvida who seemed to show up every time I did the thirty and Jigoro. I was swimming in Risky Brother manuals before I finally got them and the same for Young Franky's manuals but I still haven't seen him.
Based on the data they've collected it's about 2% for Jigoro on Elite and 6% on Expert. Mind you, for the 2% on Elite more than half the data must be from my runs and even for Expert they only have around 300 replies so I'd take it with a grain of salt when it comes to being statistically relevant. It does seem to give a good indication though. For example, Absalom dropping around 40% of the time on Expert and 20-25% of the time on Elite sounds about right. I did end up getting the secret bosses in the last couple FNs except for Conis.
@ebtc:
Yes, more stamina = higher drop rate. But it also depends on the particular FN, some bosses drop almost every time (Absalom 30 stam was one of those if I remember correctly) and others pretty rarely (Young Arlong dropped maybe every 3rd or 4th run for me).
Is there a particular reason both of you want Jigoro? I have him nearly MAX/MAX/MAX and not even used once. Other than for collecting he's completely useless! :D
Jigoro can also drop in Thriller Bark part 1 story isle on any stage.. he's a random encounter there.
Purely and solely for collection purposes. I like how the character log fills up over time.
Which is probably part of what made me pull again at the Sugo. 5 pulls starting with Ms.Valentine and Nojiko, then unevolved Marco (had evolved), then evolved Kidd (had unevolved almost ready to evolve) and then 4* Bonney (yay, a new one). Was completely confused when the first +10 poster dropped. Hadn't seen that before, lol.