Death from Above

In the 1985 movie "Rocky IV", the nickname of the Russian boxer Ivan Drago is "Death from Above". He got it after his prizefight with Apollo Creed resulted in Apollo's death. "Death from Above" would also be an apt nickname for a knight move in a standard chess game which checkmates the opposing king; the death is "from above" since it's impossible to interpose a piece to thwart a knight's attack. A knight's threat to any piece, including the king, drops down on it from the sky like a bomb, and the only way to avoid it is by moving the piece, if that's even possible. Applying this to Gargantua, any gargantua move which moves the gargantua to a square from which it checks the opposing king via its knight powers, and results in the checkmate of the opposing king, qualifies as a "Death from Above" move.

Puzzle of the day: below is a Gargantua helpmate-in-one puzzle; what is Black's helping move, and White's checkmate reply?

update: here's the solution