Archer are polyvalent and very powerfull early on thanks to their special arrow. However they don't scale very well later on and their vanilla normal attack potential is indeed low. If you want her to do more than support and actually do some damage through normal attack, you should consider picking guerrilla with sneak 5 for a 100 % damange boost at the cost of 1 ap per attack (as a bonus, it fits your theme). Archer also synergize well with bully talent because they can easily slow/knockdown enemy and can also take advantage of target disabled by the party.
Without changing anything, since you already have witchcraft, oath of dessecration is also a good way to increase your damage (50% damage for 3 ap for 3 turn)
Undead are usually resistant to poison and piercing but vulnerable to fire. Fire and explosive arrow are your best friend in Cyseal. Despite the poison immunity, a poison arrow on a burning undead can also be effective (double fire explosion combo).
Also try to take advantage of oil and ooze barrel. A well placed arrow from a scouting invisible archer can often take out several enemies before the fight has even started.
