Generally the strongest traits to power up a caster are Magical Lineage and Wayang Spellhunter. Then take metamagic feats to make your spells nasty at their original level. Fireball, Magic Missile, Shocking Grasp, Spiritual Weapon, Suggestion, and Call Lightning are all favorites depending on what I'm going for. Spell Focus and Spell Specialization can achieve the same effect as the CL boosting traits, and they generally don't need to be retrained out of due to being able to change the specialized spell.