well my strategy is basic, have my stronger units stay back until the end of the battle where I have to beat a boss, unless I trust my units weaker enough to kill the boss, usually from afar, this works very well because I got my first trainee Ross, a journeyman, unit in chapter 3 or so, and he became a level 8 pirate by the end of chapter 5, and has gone from absolute bottom of the barrel, to front line's dodging and killing everything!!!