If you keep having difficulty in keeping things alive, always use the lure and draw tactics. Yes, it's slow, but it's safe. Have your mounted units that can withstand some damage to move to the front line, and retreat bit by bit, then ambush. Once you get to spam some exp with healing/teleporting trick, you no longer need to use this tactic. Faring in arena helps as well - use the changing weapon trick to help chances of winning.
As above posters had said, FE4 shouldn't come as difficult if you know what are you facing and do the right preparation and tactics. I found that only the first chapter is difficult. Of course, assuming you play in emulator you really should spam the save state reload stuff.