[Gatts]

Actually on impossible is leveling the spirits more possible:
1. Your enemies ranks are match, overpowering and more (what gives more XP to your spirits)
2. You fight longer (you can't finish the battle with just one or two fireballs and some shots) that means it is normal, that you use each kind of active spririt twice, thrice sometimes even more (but watch out for rounds, after round 10 you get only half of XP, after round 20, you get no XP for your spirits)
3. You strongly rely on your spirits as support, on easy/normal I can easily forget to use my spirit as I do not need to, on hard/impossible this rarely happens

On easy, even if you need less XP, you get less XP as well... so if you fight efficiently you will level up your spirits with quite impressive speed.

But as Calinda said, you better know all your (even future) spirit abilities well. Not only well, but perfectly, would I say. That can prevent you from choosing not as useful levelup combinations... but knowing what a skill does is only half of the needed knowledge... how to use it in battle is the second one. The second part could be learned on this forum, as here is a lot written about the usage of the most useful spirit abilities (time back, gloth's armor...)