Board Thread:Questions and Answers/@comment-113.169.215.188-20150630180547/@comment-25999494-20150630231843

if you really want to hurry up with the levelling, 5-3 is a good choice, it's like 1-5 but for subs, with defeat instead of victory. i leveled my yuu there, doesn't take more than 2 hours to get her to her remodelled form.

if you don't feel like hurrying up with the levelling, then 4-2 is a good choice, you can tag the kill 50 transport quest and kill 12 world 4 boss quest while slowly letting her leech exp as flagship.. .. it's just that.. it will be smoother if you have at least 5 remodelled subs on your fleet while running that map.. doesn't mean that it is impossible to do it with some unremodelled subs, but you really need high torpedo power to sink those flagship CLs along the way before they do that to your little wolves.. i did the same thing as you did with your goya when she reached 50.. but i've never thought of that as a mistake.. since i don't even need subs anymore for 3-2..