Fire from the sky!

[The Wandering Scholar]

Finding higher ground on a battlefield is not always possible, infact hardly ever. You have to put them somewhere.

Same as slingers, except that slingers are actually fast. >.< Though I would imagine that the same principle applies to ballistas and scorpions as well: put them on higher ground than the troops protecting them.

With fire arrows I usually let archers fire twice at the enemy with fire to give them poor morale, panicked by fire attack, then normal arrows to create casulties and thus rout them.