Iron Giants give kick ass experience! Especially when you overkill them :)
It seems like almost anything is easy to overkill with Lulu, especially if you use focus. Certainly something to consider when i decide where to have Rikku go.
Speaking of which, currently Kimahri has 12 spheres, and Rikku has 24. That's enough to get Rikku to the beggining of the black magic path with a stopover at Lancet, something that i think might be usefull for a black mage. Or it could get her six steps ahead of Kimahri.
Hmmm, for that matter, Kimahri could make it back to the black mage area with six spheres and have six more to get some spells. However after that he'd probably progress a lot slower than Rikku would if she went that way.
Okay, Kimahri is catching up, it's 18 spheres to 31 now :)
I now have 2 Level 2 Key spheres from that spheroid boss fight (yay Overkill!) so i can now wander into the White Mage path if i wish, though i still think black mage might be better. There's also still the question of who to send back, Kimahri or Rikku, along with what to make them into.
Of course Kimahri as a black mage is almost as odd a thought as Rikku as a heavy hitter :)
Chimeras give great experience too, however they're a little harder to deal with than Iron Giants and a lot harder to Overkill. It takes a lot of focuses on Lulu for that.