I think that the best build is...
5/5 Fireball
5/5 teleport
5/5 Shatters [This may be the most important thing to you if you want to pvp, being the only stun a mage could perform.]
2/5 Illusion shackles
And last, but not least...
I have 2/5 Ethereal shield, but the point I used may be invested for something else, depends on you c:
Note:This build is FOR A LVL 24 MAGE. So there are 2 points left for your decisions. Anyway, the stuff noted upwards,(especially shatters)are really important to have.