Archers and slings are good against infantry - *if* you are hitting them from the sides or better yet back - or if they have no armor/shields. If they have good shields or armor, they can sit there and let you shoot at them from the front all day. You'll have to learn to manouver the troops better to have ranged units be effective or just make sure they aim for guys with little armor.

There is a patch in the traits thread that will help you get command stars more quickly than you might currently.

Good luck!