if i remember correctly..
class B win 10 OB before lv 50
class A win 20 OB before lv 100
class S win 30 OB before lv 150

but if we do OB till 100 battle, and we still not reach class S yet, we will force to change class to EX, right?

my fubuki already has 106 battle, and she still in class A.

what's happening?

Does your Fubuki have at least six draws? Draws don't count towards the 100 OB total.

oic..
my fubuki has draw more than 10 times..