Astrid's not really useful - the game gives you a LOT of Paladins, and she's not one of the stronger ones. And being a Bow Paladin, she won't even get much use out of the SS-level bow, the Double Bow, whose main claim to fame is being able to attack in close quarters. Shinon or Rolf's your best bet if you want a bow user - Shinon is faster and more durable, while Rolf has better strength growth. But I generally find Shinon easier to work with - something I learned when I played the game was that level ups earned by Bonus EXP always gives +1 to three stats, no matter how low their growth rate in a stat may be, unless they have fewer than three stats still left unmaxed. So this means that once units start reaching stat caps, Bonus EXP level ups become extremely valuable for boosting up the stats they don't gain as easily, such as Res for units like Ike and Haar.
What I became fond of doing was picking my units so that my final Endgame army would have a unit capable of using each SS level weapon. Ike gets his Ragnell of course, Mia gets the Vague Katti, and Mist gets to swing the Alondite around - she can't do crap with it, but I find that weapon combo hilarious. Haar gets the axe Urvan, Nephenee gets the lance Wishblade, Shinon gets the Double Bow, Soren gets Rexcalibur, a certain unique fire mage you get in Part 4 gets Rexblaze, and although she's a very subpar mage this time around I go to the trouble of training Ilyana just so I have someone to use Rexbolt at endgame. Sothe has his dinky little SS knife of course, but he can't do crap in Endgame so he just sits back where he's not in the way. The sole exception is that Micaiah gets Nosferatu instead of Rexaura, just because she absolutely murders Endgame-3 with that thing.
