Hold down the Select button before entering Campaign Mode and you'll play in Advanced Mode. You'll face stronger enemy opposition, but win more coins if you complete the mission
If you would like to erase data, press L, Select, and right on the D-Pad immediately after turning the Gameboy Advance on. The delete data option will appear, and if you want to begin again, you can.
Open Up New Gameplay Modes:
To open up the War Room, Campaign, Design Maps, and Stats options on the main menu, you must first complete all missions in Field Training.
Skip Field Training:
If you want to skip field training and get into the war campaign, map maker or etc., instead of winning every battle in field training, just go to the last one and beat it. It'll save time, and you can always go back to play it any time on the main menu.
To access the field training mission called Special Intel, first you must clear all of the other Field Training missions. When you go back to field training you'll see it in the list of missions.
Use Other CO's in Multiplayer:
To use other CO's in multiplayer, you first beat campaign and then go to battle maps where you can buy them.
Unlocked after beating Campaign Mode.
Beat Campaign Mode and use Max in Mission 4. Buy from Hachi for 50 coins.
Beat Campaign Mode and use Sami for missions 15, 16, 17, 18, then beat secret mission 22. Buy from Hachi for 50 coins.
Beat Campaign Mode. Buy from Hachi for 50 coins.
Beat Campaign Mode. Use Andy for missions 15, 16, 17, and 18. Buy from Hachi for 50 coins.
Beat Campaign Mode. Beat Mission 9 in 8 days (or less), Mission 10 in 10 days, Mission 11 in 12 days, then beat the secret missions 12, 13, and 14. You can then buy Sonja from Hachi for 50 coins.
Beat Campaign Mode and unlock all other COs. You can then buy Sturm from Hachi.
To Play as Nell:
First, you must unlock the Advance Campaign by beating the game in normal mode. Then, enter the Battle Maps Shop and buy Advance Campaign for 1 coin. Hold down the Select button and select Campaign to begin an Advance Campaign. Once you have beat the Advance Campaign, you will be able to buy Nell for 80 coins in the Battle Maps shop.
Thanks to Revolution readers Albert Cacdac and David Ahn!