I just didnt use them in the prologue.
For one Spears are more useful since they protect from cavalery and monster units like giants. I had 2 units of spears and everything else heavy infantry. Heavy infantry also seem to give you better result if you auto resolve battles, which is another reason to avoid light infantry.
By the way, I hate fighting in forest and having units blink in and out of visibility. For this reason the first thing i do when a battle start is to run to a wide, open plain terrain. (I wish there was a unit like in Shogun that can serve as a scout and reveal hidden units from a reasonable distance.)
People who like fighting in forests should do the inverse, get rid of heavy infantry and use a lot of light infantry, and by the same occasion get rid of your archers too since they have a penatry for shooting FROM forests and AT forests. This is another reason why i dislike this style of play, I like using archers.