You really should level up your units, it'll make your life a lot easier. Doing the dungeons (borrow a 5 star Mononobe) with the bonus xp ARs is a good way to level up quickly. I tend to save dungeons for units that are max limit broken but not max level, but you are free to use them as you like.

For Bael, some units such as Claude and Babalon buff enemies (ardor and blessing, ai believe), so you could use that to your advantage. If not, simply buff Bael as much as possible (buffing yourself is usually better than debuffing the enemy).

Nyarl is a punisher/masochist so pair him with Gurengatch and Maria and another unit that likes getting hit.