Fantasy / Magic
"Not all those who wander are lost." - J. R. R. Tolkien
Working magic into your fight scene is very similar to describing combat or battle scenes in many ways, but you have the added element (and possibility of serious unpredictability, depending on the ineptness of your spellcaster or quirkiness of curse or magical weapon) of spells, and magical weapons and/or defences. Before you use any magic, you have to do exactly the same as you would with a normal weapon. Work out how it works; work out the effects, understand the limitations, consider how the use of it will affect the parties involved, and whether the defences will withstand the onslaught. Other than that, it's pretty much action as normal but with a whole lot more potential for oddity, colourful lights and the occasional tactical nuclear penguin (see clip above, although Voldemort didn't use any penguins to my knowledge...)
If your characters have any powers at all you have to know their boundaries. As usual, ask yourself questions and use Mr B's 6 Ps.
- Can the characters use their powers indefinitely, and are there limits to the number of times they can do things?
- Do they have to stop and cast a spell (kinda difficult in the heat of battle) with their eyes closed?
- Do magical characters have different strengths and weaknesses?
- Are their powers defensive or offensive, or both?
- What damage can they cause?
- How would they be useful in a fight?
- Does using their powers wear them (characters or magical weapons) out more quickly?
- Can your mages wear armour?
- Can they fight at all?
- Do the characters or weapons have to recharge?
Once you've worked out their powers, limitations and characteristics, then they're the same as any other weapon really.
Whether you want to drop a magically enlarged two-toed sloth armed with a snarling granny (probably a battleaxe) into the middle of your medieval style battle is entirely up to you.