How do I atch suicune I can't seem to catch up with it?

You need charmender as your Starter, then use a lot of max replesc and go look in the wild. Bring a Pokemon that has the abilty shadow tag/arena trapor the Pokemon that know mean look ir block

Keep a Pok'emon in your party that knows Mean Look or Spider Web. Trap Suicune as soon as you encounter it, then weaken it down.
If it escapes you, don't worry; the next time you face it it will still have the same health and status effect.

You need to start off with charmender dude.

You need to start off with charmender dude.

Well to catch Suicune you had to have picked Charmander at the very beginning of the game. If you picked any other Pokemon then it will not be harder or easier to catch the alternate legendery dog (Bulbasaur-Entei, Squirtle-Raikou).
First of all you would have to have the National Pokedex. Then you will need lots of max-repels. Have a tough Pokemon at the front of your party who either has the move Mean Look or the ability Arena Trap or Shadow Tag. A very good Pokemon for catching Suicune would be Wobbofet from Crulaen Cave because it's psychic, and usually level 55, when Suicune is level 50. It is not like Latios or Latias, you do not need a Master Ball to get it (well, I think you nedd a master ball to get Latios/Latias) I caught mine with lots of Ultra balls.

First of all you need to have picked Charmander at the beginning. You then need the National Pokedex. Then you need a strong Pokemon at the front of your party, with either the moves Mean Look or Spider Web, OR you can have a Pokemon with the ability Arena Trap or Shadow tag. A great Pokemon to have at the front is a Wobboffet from Mewtwo's Cave, because it's usually level 55, when Suicune is level 50. Pack lots of Ultra balls or a Master Ball if you didn't use it on Mewtwo. Use lots of max repels and look around Route 1. If it dosnt appear Fly to Saffron and go in the poke-center. Then go out again and fly back to Pallet Town and go to Route 1. I'm sorry it will take a very long time. I finally caught mine by accident. But you will eventually find it don't worry. It's just not as easy as finding it on Gold or Silver.