Magic system design is crucial for a fantasy author. You want something that has internal consistency and preferably looks and feels different from other magic systems. By the way, Brandon Sanderson in his Mistborn saga has one of the best-designed magic systems I've ever read. If you haven't read them, do so. They are excellent.
I'm not saying my magic system is even close to his, but I did give it a lot of thought before I started writing, and I've continued to tweak it as I go along. I didn't want Dafydd to feel like a D&D style wizard who tossed around fireballs and called down lighting bolts. I wanted him to use his magic tactically and to give some thought to how he could use his powers.
In the first story, I said he could create potions, do divination-style magic and read auras. I also said that he needs time to do his magic. He can't pop off spells at a moment's notice. He needs time and set-up. As you'll see in the book I'm writing now, I break that rule, but for a specific type of magic. My husband actually came up with a way he could use that type of magic in combat, and I'm looking forward to writing the climactic fight scene in that book where he tries something new.
But until then, I've stuck with the original rules. When I write fight scenes, I have to think carefully about Dafydd and what he'll do. I can't make him sit on the sidelines while Paul does all the fighting. Basically, I've dealt with it by having him augment himself and Paul with potions (enhancing speed, strength and reaction). He uses pepper spray to good effect against a werewolf, and he's used his magic to trick and misdirect.
Sometimes I have him avoid the fight entirely and save someone, freeing Paul up to so what vampires do best. Beat the tar out of bad guys!
Of course, he uses his magic outside of combat. Aura reading has come in very handy while talking to other vampires, and I have a scene where he uses ritual magic to track someone down.
Check back next week when I'm going to post a draft of one of the fight scenes from The Case of the Lost Werewolf Puppy. You'll be able to see both of them in action. I'd love feedback on what you think of how I've written Dafydd and his magic.