Use Destiny Bond on either Gastly or Froslass, ensuring that every time your opponent takes a prize, you do the same. By the later part of the game, you will have either won (If they bring up an EX, GX, etc. and give you more prizes than they take) or you will both have one prize remaining. Then switch in Slowbro and use Walk-Off Homer, an attack that we've all wanted to use ever since we saw those fateful words; Win The Game