This is vitally important. Watch everyone's exp and levels and if there is even 1 level difference, just use Guard on the character who is ahead until the levels are corrected. End a level with one enemy alive, and use Curses and Heals and Buffs to gain exp. It's very tempting to build Nahmi to take a ton of extra turns. But this can cause her to gain exp way faster than anyone else. If she does, it's a problem.

I actually didn't even use Blitz. It's too hard to position to make it powerful and her EXP runs away from the team. I highly preferred using Knife and keeping enemies at bay. The lightning-bolt dice power-ups are enough to give her extra turns, and if her EXP is getting away, just Guard or Draw cards. In fact, with her powerful heal spell, you can use Nahmi as a tank. Move into position, Heal, if you roll a lightning bolt, Guard. Shadow can also be thought as a defensive ability.

There are two events in the story that cause level differences in the game that you have no control over. So, to correct it, you're going to have to use skirmishes and methodically get everyone's exp even.

Another tip, the enemy levels are based on your highest character's level. What you can do, is get your characters within 10-20 exp of their next level, start the next mission, and you can gain your first level immediately. If needed, you even 'level' up during missions to even out your levels. It's a little cheaty-face, but it does work. Find a hiding place, and use abilities to Bind or take away the enemy cards and just level-up like during a skirmish.

Hope this helps.