Board Thread:Questions and Answers/@comment-26154471-20150720010901/@comment-26021614-20150720072948

Serial and Mich both right. Before Celgrad, the best exp/energy ratio was Bariura's final boss, but I wouldn't be able to recommend you auto-battle that because the boss can negate leader skills rendering your team to be pretty much powerless unless you use buffs, spheres, items, etc.

I haven't beaten Celgrad yet so I can't say whether or not autobattling is a good idea, but the guide will tell you that that is the best exp/energy ratio.