destroying a battle ship fleet 40 strong

Subs and naval bombers, especially if they stay in a stack of 40.

Depends on levels, lvls 1 axis subs will loose from pan-asian mixed fleet at lvl 1 (10 of each, no subs), at max its inversed. Used dxters calculator for it.

http://dxcalc.com/cow

I would try it a bit to see what can work best, you can also create 4 different substacks attacking the big stack, I guess its less good, but cant be sure,

I cannot guarantee that this is 100% matching game outcomes, but in general pretty solid if you input the figures right.