My wizard slayer hauled ass to Spellhold by doing the circus tent, Mencar Pebblecrusher, and Improved Copper Coronet. But you don't have to buy much gear to do that.
If you're not soloing, the de'Arnise keep and Trademeet are both decent for gold and experience. Just have a few +3 or greater weapons (or Phantom Blade, Melf's Minute Meteors, and other spells that hit as +3 or greater weapons) and a few items that guard against status changes (stun, fear, etc.) are also nice, but not necessary.
The only way I know of to get her at her highest level is to get to Spellhold as fast as practically possible. When I picked Immy up in my latest game, she was level 13. That's as high as she gets, I think. The trick is to keep the party's level down where she is so there won't be such a big difference in party levels.