You should complete the Find Beastslayer quest before using Archer, to gain her Critical ability.
Charge and Improved Marksmanship can be combined for amazing damage output. Charge nearly doubles her already good base damage, and Charge provides an effective 50% bonus in addition to that. Charge should always be active when the archer is attacking, and it's recommended that you get something to increase your attack speed. Playing with a time mage can help, or using any Haste item.
If using Seitengrat, you should use Rapid Fire whenever it's ready. Each shot from Rapid Fire can deal a little over 2,000 damage. Starrain and Arrowwave are very weak in the late game, and you probably shouldn't bother with them.