I used my mage as a 'pure' mage throughout. Though he did give my DK a hand in melee sometimes, he wasn't at all trained in it. A high INT gives a good damage bonus with Shadow weapons (Oaks are good, IIRC) which helped.
Developing weapon use to a reasonable level uses a lot of skill points - as does developing spells! I would not advise trying to create a true Battle Mage, as you are likely to end up with a character who is too weak in both areas, and will probably not be able to get some of the more useful secondary skills.
