If you're gonna level your Imp to let it have all the R6 Magic Shield skills, it's gonna have all the R6 bolt skills as well, so you can skip the Sprite rebirth.
Do Tofu Bear (don't feed) (R6 Healing by default)
->
Kangaroo Rat (all Combat R6 by Lv. 34) or Mini Bear (all Combat R6 by Lv. 33)
->
Imp (R6 Shield & R6 Bolts by Lv. 56)
->
Pet of choice
You might want to choose:
- the Thunder Spaniel since you got lots of MP due to the Shields and since it has Thunder.
- the Griffin, the strongest pet in terms of strength.
- the Black Wolf, which has Windmill RA, if that appeals to you.
I recommend the Griffin out of all three. The Thunder Spaniel's Rank A Thunder isn't so appealing to me.
Honestly though, I would choose the
White Tiger (if it ever comes out here). It gets R5 skills-- Combat and Magic. Whaaaaaat.
But that would prove all this rebirthing useless, since no other pet learns R5 skills. So you would still have to train it to get Rank 5, which is a hassle. Rank 5 Icebolt @ Lv. 70 would be your goal.
Easier said than done.