There's no way to stop the rays I'm afraid. Before the game and after each goal they will randomly eliminate either 1 or 2 players from each team (including captains) until somebody scores. After every goal this can change to a different amount of players from each side but you will never be at a disadvantage because the same number of players is always removed from each team. This is a good level for Diddy Kong because, like you say, it pays to have fast players when there aren't as many on the pitch. The only advice I can give is to make the most out of the players you are left with and exploit the fact that there are less players on the opposing team to stop you. If you have a Boo sidekick make use of the extra space available to you to sneak in and do a fake past the goal keeper. Make use of 3 x Red Tortoise shells as it is guaranteed to take out all of the other team (as there will only be 2 or 3 of them) and take the opportunity to throw in a megastrike. If you have a Birdo, a well timed fake will spin away an opposing player giving you time to perform his skillshot (almost guaranteed a goal). You're captains powerup is also likely to allow you a megastrike as they will be less players to take out of action with it and give you some charging time. Do be careful of Diddky Kong's powerup though. In my opinion its the best in the game, especially on this level as it can send away all of your players that enter the circle he creates. If you're only 2 players are in shot of this Diddy Kong can megastrike 2 - 3 times before they return. Make sure you don't let this happen!
As for selecting weapons, shake the nunchuck in the same you would the remote to tackle. Hope this helps