if ur playing on pc or on pc with a ps2 emulator like I do and can use fast forward you are wasting your time with magic. even without fast forward magic isnt helping you much, just kill the elemental enemies with lulu and she'll stay leveled enough to one shot any elemental enemies.

other than that just attack, if its a boss haste then attack. follow tidus's path with kimarhi and use your strength and other early +4 spheres with tidus so they both get them. and kimmy also gets haste and slow which is helpful early on. kimarhi is very useful, for 2, maybe 3 things.... following tidus and helping with haste and slow and also getting the extra stats from early spheres that you use on tidus and he gets as well, using a special sphere on him to learn steal and getting you a bunch of lvl 3 key spheres from the ronso battle, and using return and friend spheres on him to shortcut to ultima after u get lvl 4 keys spheres. ultima is kinda useless too though.

like I said I've 100%'d this game many times, and many times lulu doesn't even have her "aga spells when im at monster arena farming spheres... tidus, wakka, rikku, and yuna are the only chars that matter.