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The problem with the tree is that it has 100 defense. If this is a universal defense, and not purely physical, then burns will damage it for a very long time. If the defense is only physical, then there is at least some hope of defeating it (without damaging the bark) in two cases:
1) Someone very, very lucky and with magic damage on their attacks. Among the player characters, only the elven twins and Ayrlis fit this description.
2) A hired priestess of Luxuria.