Freeman: yes, character balance in ST 2nd Squad sucks. But it's certainly not impossible to do one-life finishes with Angel or Lulu, I've done it with both (but usually just get one-credit finishes instead). Leon is #2, I'd say the best character is Toy because of the awesome destructive power of his bomb. It destroys bullets too like Leon's! VERY useful on the last boss.
Last boss is hard but manageable. Just save all your bombs for when all the satellites come out, and then be prepared to do a LOT of jumping. Gotta love that slow-mo!
My high scores:
Shock Troopers 1 - 29,574,600
Shock Troopers 2nd Squad - 14,145,770