Shadar is extremely easy. Just save a crapload of MP recovery items and spam Mornstar continously at Shadar,usually he'll go after you completely avoiding your other party members for the first two fights so just remember to always run and spam Mornstar,pretty useful and getting a good defensive monster too since his magic can be extremely strong. I beat him on the first try at like level 46 by just spamming Mornstar and MP pots while Swaine and Esther just attacked him with either Bone skeleton for Swaine decent enough level and just switched from time to time to use Psyche Up and Esther whatever you want to do with her
I recommend buying it. The game is absolutely amazing and you could always catch up with your other games in the future. The game gets pretty boring after you beat it so just buy,beat it,play the other games,and come back to it for the post-game stuff. I plan on replaying it in the couple months because the game is so beautiful and was one of the best starting experiences for a JRPG in my opinion
I recommend training Marcassin to level 99. He's really useful for hard bosses other than that,yes he's useless,Swaine skill is extremely useful and besides Marcassin looks lame and sounds lame in my opinion. On the other if you're going for a full-powered party than go with Marcassin.
I feel you on AI,I wish it was similars to Final Fantasy where it switches everytime to different character since spells take time to load p,but doesn't do that only for pell that take a whole animation on the other hand the game can get boring,it's. more of a JROG to st beat and replay in future in my opinion although its a generally nice game like you said,great artwork,artwork,a nd epic soundtrack,The only characters I truly loved we're Oliver and Drippy too.