Suicune

You can use a masterball if you still have one. Or use a move that will keep it in battle, like Mean Look, or attempt to put it asleep right away.

Well I recomend weakining him and try throwing great balls or ultra balls. It should work. Hope this helps

Just have some Pokemon knows how to sleep or don't escape in the battle and use that when you saw Suicune again.. Use some great and ultra balls.. Because if you use masterball it just a waste because you can use that for the stronger and rare Pokemon like Ho-Oh or Lugia..

The best way to catch him is to buy a lot of ultra balls, have a high level wobbufet in the front of your party, and Fly to Route 10. Check the Pokedex to see it's location and if needed repeat. This may take a while but it works perfectly.

Well I looked up in the book for the game and it says you can only catch Suicune with a masterball

breed a pokemon with the Master Ball in its hand and u will get another 1 when the egg has hatched so then use that o carry on using that teqneke

i reccomend hurting it each time you find it until its realy hurt then find it again and use a master or ultra ball, but the best way to do it is to use mean look on it

Well, Suicune avoids you at all costs and if you try to chase it, your chances are slim to none. I too have seen Suicune twice, but I had already made the mistake of wasting my Master Ball on my Zapdos, so I'm pretty much out of luck, unless I follow this;). I am currently searching for Suicune, and I have a lvl. 49 Golbat, and a ton of Ultra balls. I have seen Suicune twice in the same place purely on dumb luck, and I saw it in that small patch of grass that you find Tangela south of Pallet Town on your way to Cinnabar Island on Route 21. Heres what you do: Get a fast Pokémon with either Mean Look, or with a sleeping move(not Yawn) and put it at the front of your party. A Trapinch would work, too, because of it's ability, Arena Trap. Make sure that the Pokémon of choice is lvl. 49 or lower(any higher and Suicune won't appear with the Repels). Use a Max Repel. Also, stock up on Ultra Balls(50 ).Check your Pokédex to see where Suicune is. Then Fly to Pallet town. Surf to the previously described patch of grass. Check the Pokédex to see his location. Repeat this until it is near you. When(or if) it appears, immediately use a put-to-sleep move, or Mean Look. Damage it until red health, and start pelting it with Ultra Balls. DO NOT POISON OR BURN IT, or it might die, and you won't be able to catch it. Paralysis works best. Repeat this, and you probably will catch this. In my LeafGreen game, I DID catch Suicune, and I'm hunting for it currently in FireRed. I really hope this helps.
-SoulRaven