Well first you need to work out how spells are cast, and what the cost is ( being able to let off any spell whenever you feel like it is too Superman)
For instance, my guy mutters his spells ( saves on hokeyness). The cost is either someone else's blood, or in essence his own -- his own strength. If you know the right words, and say them the right way, bob's your aunty. If you say them wrong you get something else ( he once ended up with an unexpected elephant, because he was drunk and slurred).
You need a system. Once you've worked out what it is, you're laughing, because that system should give you enough ideas of what you can and can't do. And of course each guy will have his favourite spells. Mine likes to melt eyeballs.