@shoopi - so it's as if white checkmated, then played another move right after?
Yes that's exactly right.
@roopi: Right, I remember that one now :)
It looks like -1.Nd5xRf6, but I don't see the purpose of all the pieces. Bb3 is promoted, sure - but that could have been Pf7 straight down. Not sure why it matters.
That's the correct solution. And the purpose of all the black pieces is to prevent the option of retracting f5xg6.
Checkmate in -0.5 moves:
I bit my tongue on OP but can't do it twice. You don't retract the King first when you castle. Nor do you move the Rook first when playing forward.
*STOMP STOMP STOMP*
Oh, fine...