How to remove stubborn stiff acne?

I'm 23 now, and I've had facial acne eruption occasionally, though now it has quite slowed down. But something that really concerns me is a hard, stiff, thick, painless, quite big acne that developed on my right cheek and don't seem to go away. There's nothing under it, and that bump has been there on my cheek for over 4 months now. Anyone know about it? and how to get rid of it?