not the best help in the world since this Is quite the grind, but I found the Spellpunk Library was the best place to grind out levels after you've already completed the game once and need to knock a few more out. just skip the cutscene in the beginning and also skip when Cali talks to you and refuse to enter the first book. take a left and find Flynn and push the bookcase to reveal a chest with a spark lock and solve it, the locks were giving me over half a bar per completion.
also don't forget to open the chest, the chest above Flynn you can reach by bouncing on the chair next to him and also the one in the top corner behind the pushable shelf near Mags as these have a decent chance of giving portal rank experience as well and they are all right next to each other. than simply pause, scroll over to map and relaunch the level and repeat as needed.
it's such a tiny level and there are no enemies to get in the way so it's pretty easy to just get in and redo it a few times, not particularly fun but leveling far beyond the necessary level to complete a game usually isn't.