Yep Mist is a top characters but very weak until she rank up...until that Rhys is a better unit, after the rank up no discussion possible Mist is way better.
For the Magic from what I remember Soren is the best magician in the game.
Lucia is good, she has better defence and more hp that other swordsman, but she is not as powerful as Stefan for sure.
@ Shoot that Way: never tried it, I just completed all Fire Emblem who where localized (= not so much sadly...).
F.E. Radiant Dawn was quite tough too.
And yeah you can loose Astrid very quickly, you really have to hurry.