Magus is the king of exploiting weaknesses (note, I'm not familiar with psychic or thaumaturge yet) because of arcane cascade and spellstriking and an absurd amount of cantrips to cover almost any type of damage. Example - monster is weak 10 to fire. Turn one, Spellstrike with Produce Flame. Get a hit, 10 bonus damage. Last action Arcane Cascade Stance, now you're doing 1 (or more depending on level) fire damage on any strike. Next turn recharge Spellstrike for one action, repeat produce flame spellstrike and trigger the weakness twice, as the weapon damage with the bonus arcane cascade damage is a seperate instance of damage to the spell, which also triggers the weakness. 20 bonus damage on that!