Illusion is extremely open to the user's creativity. If you keep in mind that nothing you illusion is real, there's a lot you can do. Illusioning crippling pains should definitely be avoided, or you risk powergaming reports.. but really, it's mostly about deceiving the senses. Phantasms skip the senses and more directly affect the mind, thus are harder to do. Always remember that illusion is a support magic. If you're trying to illusion fire, swords and anything else like that, you have to impose limits to ensure you're not taking it beyond the nature of illusions.
That's not to say you can't use it in combat, but you'll have to think carefully about your actions if you want to stand a chance of surviving, without resorting to powergaming. An example of this being taken from personal experience, a sparring match. Unarmored mage against a fully armored and well trained knight. By temporarily inhibiting my opponent's sight (through the use of bright lights and dark mist) and harassing them with undesirable noises (the ringing of metal on metal echoing through their helmet), I was able to force a surrender without either party swinging a sword. (Though if they begin to question illogical illusions, you'll find them no longer working!)
That being said, I find it far more enjoyable illusioning lights, butterflies and rainbows to entertain children. Not everything has to be about combat. I know numerous people who've gotten enjoyment from that sort of thing, with a traditional magic tricks spin on it. Creativity is key!