1) You can use the MAster Ball on any pokemon.
2) It is best to use the Master Ball on Tyranitar.
3) IT is easier to use the following cheat:
1. Have first pokemon use the Master Ball
2. HAve the first pokemon select the Master Ball in the items menu. Use the X or Y button to change its position with another type of pokeball.
3. HAve second pokemon perform any action,
4. The pokemon that the master ball has been used to snag will be caught, and the master ball (and the ball u switched it with in the items menu) will still be there, so you can use it again.
***NOTE: This only works in Colosseum, NOT XD.