I don't rememeber wich lvl I was when i defeated Liquidflame... but i remember i went to get Shiva after defeating him... Always is better the elemental spell than the elemental esper... just make sure u have a powerful mage, and increment his magic power with his equipment to make more damage by attack...
I almost never use espers... maybe just Golem sometimes when the attack power of some enemies is to high...
Anyway... i always get them all... all the espers in ur menu look great xD...