Don't overlook the fact that you can use Augmented Casting the other direction as well - quicken or twin one of your lower-level sorcerer spells by expending a 4th-lvl Wizard slot, for instance.
The Ring of Wizardry 4 is of course the most optimized solution... and if you're 15th level or...
Consider Toothed Tentacles (Lost Empires of Faerun p35, 2nd lvl), or Thunderlance (Spell Compendium p220, 4th lvl) if Forgotten Realms is not available.
It is the action of the monster moving out of reach, while the monster is focusing on a different action (in this case, the action of attacking a new target), which provokes the attack of opportunity. If the monster is focusing on not provoking (ie, the Withdraw special movement action), then...