Mario and Luigi: Superstar Saga - GBA/Cheats

From WikiCheats

Permanent Red Pepper Status

Red Peppers increase the power of whichever character they're used on in a fight; this normally lasts only for a few turns, but you can exploit a glitch in the game to have it last the whole battle. Use a Red Pepper on either Mario or Luigi, let that character get knocked out, then use a 1-Up Mushroom to revive them. The increased-strength effect will now last until the end of the battle! This isn't particularly worthwhile for lesser enemies, but for tough boss fights, it can be invaluable.

Infinite Coins

1. Shrink Mario with Luigi's Hammer 2. Make Luigi jump on Mario. 3. During the Luigi's jump, make Mario jump under Luigi. Note: Luigi will lose a HP for each coin you get. But nothing to worry, you'll get enough and more coins to buy Mushroom to renplish Luigi HP.