Well, I'm at levels 16 for four of my characters, but that's only because I did some leveling in the Lhusu Mines before. The imperial soldiers inside the Leviathan give pretty crappy experience (compared to the skeletons, which give 90-130 experience points). If you have a save from before entering Ondore's estate, I'd recommend going back to it if you feel you need to level up. You can also use the alarm strategy, which can also give you a great chain. What level is (SPOILER)Ashe at? She came to me at level 16, though I don't know if the levels of new characters vary depending on some factor.

As for the boss...I actually have to go face him right now.