Instructions
Buy Repel at any of the Pokémarts; open your inventory and use it before walking around in the grassy areas where Suicune dwells. You need your first Pokémon to be at full health in order to prevent Suicune from escaping; using Repel will ward off other Pokémon that can weaken your Pokémon before the battle.
Place a Pokémon with a blocking move at the top slot of your party. The best bet is to use a Golbat that has learned the Mean Look special ability. This is a Normal-type move that prevents the target from fleeing in battle.
Use the Mean Look move as soon as the battle starts in order to trap Suicune in one place. As long as your Pokémon has a higher speed rating than Suicune, it will attack first and trap the enemy.
Weaken Suicine with attacks until its health is in the red; then, use an Ultra Ball to capture it. If you have saved a Master Ball, use it for an easy catch.