You need to use a custom script line to create the lightning effect because the standard GUI one doesn't allow you to add Z offset, take a look at the line I used in my Wild Energy Spell
Custom script: set udg_WE_TempLightning = AddLightningEx("CLPB", true, GetLocationX(udg_WE_TempPoint), GetLocationY(udg_WE_TempPoint), udg_WE_EnergyHeight, GetLocationX(udg_WE_TempPoint2), GetLocationY(udg_WE_TempPoint2), udg_WE_EnergyHeight)
The first part "CLPB" is the ID for the type of lightning in this case it is Chain lightning Primary, I wouldn't worry about the true part, the next two things are the X and Y co-ordinated of the start point, the third is the Z offset: I got the Z of the location the lightning was and added extra height onto it so that the lightning effect would leave the ground., the next three are a repeat of these but for the target spot.
Hope this helps