Without having played any other mage classes, im gonna take a bet Necro is the best solo'er.
Ive just dinged 27, and the further you get in the game, the more you learn to combine his spells, the better a soloer he gets...
For instance ive soloed Gustfeather as level 26. He was blue^^ to me, and i won about 75% of the time.
All my spells are adept I or below, except for Dust blast adept III, equipment is yellow-orange.
The trick is to take it slow, let your pet build up agro, keep it alive with heals, use you DoT spells well, and have plenty of power.
That way you can solo almost any foe...
The problem comes when the enemy is in groups, but thats another days lesson...