Offensive Spells

Hail, fellow Knights!

I'm considering introducing a hag in our mythic adventures, which means, at least for me, introducing spells. Defensive Spells and Utility Spells are easy and intuitive enough. I'm wondering how would you rule things put for Offensive Spells ( i.e spells aimed at hurting people )?

Right now, I figure I should treat them like brutal weapons with the Slow modifier ( to account for casting ) that bypasses armor value ( because, magic! ), and call it good...

For example, the hag has a spell that makes brambles and thorns grow inside one victim's belly causing 2d10 Slow damage, bypassing armor. Should the victim die from the spell, it would burst in an cloud of viscera, thorns and bones ( Yeah, I know... I'm that kind of GM... ) causing 1d10 Blast to everyone surrounding the victim.

How would you rule things out in your games? Is my approach interesting? I'm open to your constructive suggestions.