How can he edit fire arrow to chain lighting? (or what do you mean?)
I would make it like:
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If
Shaman's mana >= <Spell mana cost>
Then:
Create dummy
Order dummy to cast chain lighting
Set mana of shaman = mana of shaman - <Spell mana cost>
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Note 1: don't forget to remove memry leak (location)
Note 2: You can make for shaman a skill, that will do nothing, with a description of chain lighing that will be casted by dummy.
Note 3: This spell is MUI, causedummy needs some time to cast a spell. Use local location for the point where dummy will be created.