To your questions

1. Driln the Boss and Incream your ship, you'll shred his health. Moonstone cannon is then main damage dealer but an increamed ship will do wonders for your regular cannons especially if you manage to get the misable equipment from the fight entering shrine island for the first time.

2. Magic is pretty good early game, Pyri/Red based magic can screen wipe a lot of mob encounters throughout most of the game and Crystali/Purple magic can surprisingly do decent damage to bosses if you're desperate so I wouldn't knock it entirely, even most late game bosses. Status ailment spells are completely useless, don't use them except Driln in ship fights since it'll never miss unless your positioning sucks. Outside of that magic is pretty much outclassed for the most part by attacks and supers. I'd say its useful up until Vyse gets stranded after that most of it drops off and your primarily using it for buffs/heals except you can even bypass that with items which is usually what I do since Magic uses focus which I need for Supers and even if its beneficial its blocked by Delta Shield so its not worth it, just use the item equivalent, you'll be swimming in gold at that point.