Need help with threat mob scripting

[yashirotcm]

Im trying to make a Lua mob script that should cast an specific spell on the 2nd player of its threat list and another spell on the one its 3rd, im still new to Lua and wouldnt know how to measure threat, im assuming the command pUnit:GetThreatByPtr() is used for this but i dont really know its usage, so if someone could care to give a small example i would be deeply appreciated and +rep