Golden Bat isn't generally a high scoring ship but he keeps rank low. Gain's bomb is great against flamingoes, bomb the castle and then just plop 2 of them right above where they spawn. I almost always have +2mil after them flamingos with him (suiciding twice on stage 1 boss). Gain is also really good against madball (stage 2 boss) because if you land a full bomb directly centered while he's opening up you can get another +1 mil. Also, that opening bomb on stage 3 (technically right before stage 3) gets you a nice pile.
Good luck!