Thanks
How to get killer king award??

You can also get this achievement by castling checkmate; this counts as a king move even though the rook is the piece doing the checkmating.
I've tried once but it just counted for castle victory

I did It against a bot (Martin) but it is not showing. Why? may I know the cause?
it ususally doesnt give achievements when u play bots

2 rooks/ rook and queen. Check the king to the edge of the board, so you have it on the 1st or 8th ranks, or the a- or h-file. Run your king over there, and position between the piece that could deliver mate and the enemy king. Push that piece behind the king. Boom! Killer king. (this explanation is terrible. Just look below)
Discovered checkmate.