Tremendously one sided. I don't think i need to mention that Kid Buu has no inherent weakness other than pure spiritual energy, and that on a whim he can obliterate a planet. He even beats Luffy at his own rubber game because he can regenerate, multiply and detatch himself into pieces. He can fly, teleport across space and dimensions, transmute matter at will, absorb you and so on. Luffy's only game is rubber and some haki. "rubber" is perhaps the least of Majin Buu's abilities and he is better at it than Luffy. He eats Luffy (literally), then destroys One Piece World, then does the same for the whole HST. Luffy doesn't have a chance in one million to win. He's literally incapable of killing Buu even if he stood still.