it is possible to level up REALLY high what i sugest is that you level up characters around Lv 20ish by end of disc one.
I think it is possible to get quina's abillty Lv5 Death. If you do have it go to the married moogles as Zidane and then go up the ladder and into the open, run around till you get a grand dragon and use it against him ^^ about 1000-3000ish Exp there 4 u ^^ For Garnet (Dagger) and Stiener, it will take a while longer. Start of disc 2 you'll get 2 fossil roo, run around there, thats where the stronger monsters are. Get Dagger to an Mp of 94 (i think) and then you can summon SHIVA!!! ^^