On any level that has an armory and the boss is sitting on a throne/gate/ect., bring any unit with a very high defense (Wallace or Oswin are the best). Move this unit to attack of the boss and let the boss attack him. Heal our unit whenever you need to but DON'T kill the boss, disarm your person, stand back and wait for the boss to heal, whatever needs to be done so that the boss lives. Eventually the boss's weapon(s) will break, then attack the boss and let him heal, your units will gain some descent exp. for dealing damage to a boss. Let the boss heal after every attack. Your people's weapons will break too, just go over and buy more, this will take a lot of time and money, but you can raise your people safely and sort-of quickly.