To find Relicanth (a hard to find Pokemon), dive around Sootopolis. Hold the A button while moving over the seaweed things. You MAY find Relicanth.
If your Pokemon is about 20 levels stronger than your opponent, hold A while doing damage to it. You will probably give your foe a One Hit K.O. If you want to keep your opponent in red health, hold Right on the Control Pad to hopefully prevent your opponent from fainting.
When your opponent attacks your Pokemon, press Left or Right on the Control pad to hopefully dodge the attack. You need perfect timing for this.
When you throw a ball, press A repeatedly to have better chances at catching it.
Just to let you know, some Pokemon may be colored weirdly. I found a blue Corphish instead of a yellow one. There are also very shiny Pokemon too! Shiny or differently colored Pokemon are very rare. You mostly find different colored Pokemon in the Safari Zone. Look all over the Hoenn region to find shiny Pokemon.
One more thing I need to say is that why do people think Treeko stinks on ice? In have a Treeko, and it KO's like crazy! Don't give judgements to a Pokemon just because you don't have it. Does that mean that I can just say out of nowhere that Mudkip and Torchic are horrible too? All you Treeko fans, please support my comments. Thank you.