OK, I got him.

Interestingly I chained Mom Bombs this time. It took exactly the same amount of time, because there were fewer Oses in the zones I was going through last time, even though they were easier to fight.

I also discovered that you can use a save crystal inside without affecting the count. Also chaining isn't necessary, but it helps me keep count easily.

If nobody has Bestiary issues, this thread can be closed.