Board Thread:General Discussion/@comment-27011882-20151026041425/@comment-749631-20151026103513

p.s. As after certain stage your subs would go on level even if you don't want, so don't overdo that.High retreat rate and low win rate are expected if you use single submarine method.

Another alternative way to level your sub would be 2-4-A with accompanying 5 other ships to tank away part of damages done to the fleet. Not as good to level sub as you constantly need to repair them, but it also help boost the level of other ships in your fleet.