My only character was a mage. I wanted to be a warrior mage, so I never added points to healing. Honestly, I wouldn't even bother, especially if you are planning on having a healer in the party. Anders has that specialty that heals the entire party and revive them.

As to make an unbeatable mage, there really is no real answer. Most people will reach level 20 or 21 once they beat the storyline, and that's enough for about 3 skill trees. I made my character completely into an attack guy, so it was all spent in elemental, the rock powers, and force specialty. I did decently well.

To make an unbeatable mage, you would need to add enough points to constitution. magic, willpower, and also some points to w/e the one that raises defense is. The skill trees are harder to say.