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Wizard Shield

       Usage: cast 'wizard shield' : cast 112
      Effect: AC -20 per cast
        Cost: 40
    Duration: 1/2 hour
Casting Time: 3
      Target: Caster only

You can cast this spell up to 5 times, for a total of -99 AC. The upkeep
cost for maintaining this spell increases exponentially with the number
of copies you are maintaining.

The classes that get the spell wizard shield are:

bard       : Level 109 (average)
wizard     : Level  43 (excellent)
druid      : Level  57 (excellent)
necromancer: Level 105 (excellent)