Go to the BP_SmartAIComponent and create a new variable, set its Variable Type to Float and set its name to Damage Multi then set its default value to 1!
Next copy the image below adding in the highlighted nodes, locations at the tops of the screenshots. Note there are 2 places in the Range Attack function this must be done, that is why there are 2 images of that function.
Next go to the BP_MasterProjectile and create a new variable set its type to Float and call it Damage Multi and then set its default value to 1! Then copy the images below, locations at the tops. There are 2 in the Range Attack function because you need to do the changes in 2 places.
Next in the BP_SmartAIComponent create a new function called Change Damage Multi and add a Float input called Change Amount then copy the image below.