Through natural progression, you will likely be around Lv16-18 during your first playthrough. This means you'll likely hit Lv20 before finishing up your shorter New Game+ run.
If you're like me, and wanted to hit Lv20 in Playthrough 1 in order to have max stats for the Secret Boss, then there are two grinding spots I recommend:
- The two Dullahan enemies before the Shrine of Seasons entrance in the Arcane Fifth Wing are worth 200xp each and quite a bit of money.
- The big mob of standard enemies near the Portal Gate in the Winter area (where you fought a Nexus Guardian). As you can just jump in and out of the portal to respawn the enemies (and they're really easy to beat), I found this to be the best place to farm out a few more levels.