Which is the best spell to get for my pet to use in battling?

There is no 'best spell' -- you change your scrolls depending on the situation/opponent. You might want to take a look at http://subeta.net/forums/view/452816

well ryan is right about that. but one scroll that many battlers find essential is a healing scroll. I use the soothing current scroll, it heals 6%. there are other healing scrolls that heal about 10-14% i think but those are more expensive

Thanks so much the both of you. I think i will get a healing scroll. makes lots of sense to have a healing scroll.