I've been using the base stats on class changing to give growth screwed units a bump. On my first play through, for example, my Belyth only had 12 defense at level 30, so I put some effort into heavy armor and save abused promotions until I was able to make him a fortress knight for a 7 point defense bump. Making most of your mounted units holy knights fixes low resistance with minimal investment in faith is another good example.
As far as getting levels for low leveled units, make the character you want to level an adjunct of someone who can solo clear a map and give them the exp gem.