This game is rather opaque. The description says bayonet charges are a thing, but I didn't see any such options for that.
I played on normal, and what I got from the first several maps was that it worked to just stand a little outside where the muskets seem to fire and then wait for enough hits to land. Then I got to the map with the description saying the enemy would do a "frontal assault" and spent a really long time just moving the troops around to avoid artillery. Once my patience ran out I tried just placing the troops as close as the cover art gif shows and my troops started running away before they could shoot much. My troops had double the enemy's morale when I started to close in too.