Airplane Level Glitch
A section of mission 5 of "Metal Slug Advance" has the player piloting a plane. There is a glitch in this level that can allow the player to easily complete it. Press and hold Up until you reach the top of the screen. Press and hold Left after you are at the top of the screen. No enemy planes will appear while you are holding down the Up and Left buttons. You will be able to fly through the entire level without needing to fight.
Entering Codes
The Action Replay is a cartridge that fits into the slot on the Game Boy Advance. The game cartridge for "Metal Slug Advance" is inserted into the slot on the Action Replay. A code menu will appear when you turn on the Game Boy Advance. Select "Enter New Cheat" from the top of the menu to add new codes. The master code should be entered first because it is required for the other Action Replay codes to function properly. The master code for "Metal Slug Advance" is 2286524f-8e51fd40-3112d648-b66140ba.
Action Replay Codes
"Metal Slug Advance" has four different codes to make the game substantially easier to beat. Gain infinite health by entering da2f5ad7-2313c12e. You will still be hit by attacks normally, but this cheat stops health being lost. Make your character invincible by entering 6c869fe3-3316a68f. This code is similar to infinite health, but the character will not react to being hit as well. This is similar to the temporary invincibility acquired when leaving a vehicle. Gain an infinite supply of bombs by entering 961bc0cf-057174dd. Jump higher than normal by entering 2ff96ea5-d5206750-a0d99078-6883b588.