Personally if I had to play a prank on my arch nemesis right before the biggest game of the season, it would have to be very low key and discreet as everyone would be at the upmost on guard. One prank or mischief I might pull is shortening each and every one of their sticks by and inch or two. Hopefully that would not notice until the game started and that slight difference would be enough for them to miss all of their shots. I would also put tape on the bottom of their skates so that when they step out on to the ice for warmups they would fall onto the ice and every one around would laugh at them leading to psychological damage right before the game. This would probably make them too anxious and again they would miss all of their shots. Maybe during the game while they were struggling, I would even go up to them and let them know it was me who was pranking them.