There are many rare candy cheat codes for Pokemon Emerald. They are very useful in the game because it allow you to gain level instantly which in return will makes your Pokemon monster gain more ability and skills.
Here are the GameShark codes to activate the Free Rare Candy cheats.
First you have to enter the Master Code: B749822B CE9BFAC1 A86CDBA5 19BA49B3
Once done, on the 2nd line, You can enter in: BFF956FA 2F9EC50D (Rare Candy Codes)
It will appear in your item storage. You can find it by going to your PC from the Pokemon Center.
Here are the Pokemon Monster Cheat Codes, once activated, walk into the grass to run into them (Remember that the Master code: B749822B CE9BFAC1 A86CDBA5 19BA49B3 always have to be on the 1st line for any cheats to work correctly)
Mewtwo Cheat Codes: D027D341 E184CF59
If there is any codes that is not working, please let me know in the comments below.