How Would I Approach Scripting This Spell Fighting Sytem?

So I’m making a game where you buy or collect spells and use 12345 to use them in combat. There will be single target as well as multi target spells.
Your pets will give things like: Crit Rate % Crit Damage %, as well as a normal damage amount.

So when adding these modifiers I’m not sure how to go about it so that the pets and the fighting works hand in hand.
All around, what would you script first and yeah just apply the functionality?

My main curiosity is the multiplier %’s from pets, where I can set up an easy way to continue to add more as needed and apply those to the combat system as well.

This is my first time approaching something like this, so anything helps, and I’d love to hear an all around idea of what you would in general even.
Any ideas related to a system like this as a whole you think would help are also appreciated :smile: