Gunslinger Build (Lv. 10-200)

With the new KMST Battleship updates, would it be wise to max Battleship after Battleship Cannon?

It's all about preference. Personally if I was level 120 with the new KMST update, I'd sitll leave it at level 1 for a while. I'd rather get skills up higher first before making my ship last longer. Your ship doesn't even break that often once you reach 130 unless you have poor control or are unlucky. And even if it does, 90 seconds isn't a long time.