[twicetwice]

You can definitely get pretty creative and interesting—for example, transmuting a pile of primed hand grenades into a handful of confetti and then tossing it over your enemy right before the spell wears off. That said, you're right, it's highly likely that boring strategies would dominate. However, the Harry Potter magic system is so fast and loose that you can always make up some new spell to cancel out whatever OP strategy; the author has almost unlimited leeway.