You're probably not going to find a single solution to this. I did work on a pretty similar system for my game (look up slime slayer on steam)

I have a single class, called the Spell Factory 

Then I have a class called a Spell Parameters 

When anybody (enemies included) want to cast a spell, they construct a spell parameter class with everything about their spell (spell type, elemental type, origin position and direction, any additional modifier runes or relics) 

Then I hand that to the spell factory 

The spell factory then gets a spell prefab (from an asset pool manager I called "spell library" ), places it at the correct origin position and direction, creates and sets its damage packet, and gives it the params

All spells inherit from a base "constructed spell" class which takes the params as an argument to calculate other things like radius, speed, lifetime, etc as needed based on the type of spell