So Mewtwo is hard to catch...

[Pokechamp]

So what do I do? It's in Firered, I used a master ball on Suicune.

 

[Jack]

I would just bring a Pokemon who can paralyze it, a Pokemon with False Swipe, and some Ultra Balls. After the 10th turn, use Timer Balls instead (assuming they're available in FireRed) and hope for the best.

 

[Pokechamp]

Thanks! I'll try now!

 

[2071]

Just as Jack said , try to inflict a status problem on the Pokemon before catching Mewtwo because it increases the rate of catching it . Paralyze being most useful , avoid stuff like burn or poison since it can kill it . Using effective Pokeballs are also helpful

 

[3905]

I prefer using a sleep inducer, but paralyze works as well. Carry a lower leveled pokemon that knows Quick Attack to slowly lower damage, I tend to do that a lot and it works. Plus Mewtwo is one of the hardest Pokemon to catch in the game.

 

[SquallFulton91]

Bring a high-level Parasect with the moves Spore and False Swipe. Use those moves to put Mewtwo to sleep and bring its HP to 1, and repeat with Spore if necessary. Keep chucking Ultra Balls, and if at least 30 turns have passed, switch over to Timer Balls, which you can buy at Two Island after obtaining the Ruby and the Sapphire.