Oh my goodness yes that is a tricky, annoying problem. One never knows whether the person is actually aware that the music is too loud or simply doesn't care. There is a small chance that the person has a hearing problem, or is simply high on life and otherwise a very pleasant, reasonable person. Here are my suggestions:
1. If there is a body corporate, managing agent or caretaker for the unit, perhaps mention to them and try to stay out of it. They can say that "people" are being inconvenienced, without mentioning your name.
2. Ask people around you if it bothers them too. And if so, write a civilised, polite letter on behalf of all of you and pop it under the offender's door. Or let the biggest guy write the letter
Strength in numbers, try not to tackle it solo. And you may make some new friends...
3. Find a cheap set of headphones and leave it in front of their door. Hint, hint and anonymous
4. Start playing your own kind of music loud and point the speakers in their direction as slickstick says. This can go two ways though: the person realises that the music is too loud and turns it down, or simply amps it up even more. Your music will then bother all the other people around you and you become the problem. And so it escalates. This is last resort.