Waves 1-4

I suggest you take as many outposts as possible, before enemy wave attacks your main castle.

If you are having difficulty passing the early waves, try upgrade Archer units (elven hunter in particular), and spam a few of them early to mid game. Their range covers almost the entire battle field and can do wonders.

If you have difficulty against a particular enemy unit, study up with the Scribe( the Bestiary) and see what counter works the best.

For example- use Shield Knights against the Archers in Wave 2.

Just don't let anyone kill your hero you should be fine.