5.. Ng5+!
Black is lost if he takes on g5 with the bishop or moves the king.
For example,
5.. Ng5+ Kg7
6.. Nxf7+ Kf6
7.. Qg6#
or
6.. ... Kh7 (or Kh8)
7.. Qxh6#
or
6.. ... Bg5
7.. Qxh6+ Kxf7
8.. Qh7+ Kf6
9.. fxg5#
In the other hand,
5.. Ng5+ Bxg5
6.. fxg5 Black is lost with any play
RE: Chess problem 68 / Schachaufgabe 68