Once you get to Proof of Existence, you can go back one screen and fight the Assassins and Snipers that form there. You should get around 8 EXP for them. If your level is high enough, you can add the Nobodies at Addled Impasse (Saix's encounter), and Havoc's Divide (Luxord's encounter), which are located inside of Proof of Existence, in addition to the Nobodies one screen past.

Or you can kill the Creepers at Naught's Skyway, for a nice 20 EXP, assuming your gague lasts long enough in Final Form to survive the trip back to a save point, or your level is high enough that you survive the encounter with the Berserkes at the end of the Skyway.