Against the oak sapling: Enter combat before they can see you. Start combat, and damage them with a ball spell near them. In the next combat round, you usually cast buff spells. When they come to you, retreat into the anti-magic area to fight them (be sure they can't cast on you as long as they are in the corridor). If necessary, you can retreat to the blue marble door. From there, you can attack them with spells, and they can't cast back.
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