What happens is that Kid Buu dies by SSJ3 Goku's hand when he does a massive universal Genki Dama(Spirit Bomb). Goku nearly cannot maintain this level as he drops to normal before he can launch the bomb, but then Mr. Satan calls to the Earthlings and has them all lend some of their energy to Goku(at this point Earth has been wished back). Goku immediately becomes a SSJ3 again and powers up the bomb, launching it to Buu who tries to deflect it. Goku says, see you when you reincarnate again Buu, then finishes him. Totally and utterly vaporized.
That's the end of the series pretty much, there are about 10 more episodes afterwards dealing with the reincarnated Buu in the Budokai tourny, but it's the end of the series by then.
So that answers your question as to how Buu gets it in the end. He can't handle the immense power of a spirit bomb powered by all of the living things in the universe. Bye bye Buu Buu.