You kill all the bosses, then are teleported to the lobby, noones there, no boss doors.
Have the lobby guy say "You've defeated all the bosses? Heh, yay you, but you're not getting out, I'm afraid I'll have to kill you"
You: "But why?"
Lobby guy: "You see, this was a government project to have the scum of the earth face off against the bosses"
You: "What, What are you talking about?"
Lobby Guy: "Oh come on, don't you remember?"
You then go to a flashback of you killing a little blockhead (Teleport you)
Then tele you back,
You: "But... But...."
Lobby guy: "Prepare to die"
The boss is up to you, I just made the dialog.