OK, you have several steps, which i'll explain by the correct order. Please follow them closely or they may not work:
1 - Make sure you have access to Campaign(normal or hard) and War Room. Go to Design Maps. Fill the map with plains, then, counting from the upper left corner, left to right, deploy SIX INFANTRY units. Now, second line, directly below the infantry units, deploy SIX MECH units. Same procedure, only this time deploy SIX ARTILERY units, directly below the mechs. Finnaly deploy SIX MD TANKS under the artilery and save your map (my suggestion: edit mode).
2 - Now go back to the Field Training and play the level called "Naval Forces" and lose. Yes, that's right, LOSE. It's very easy, just obey Nell for the first day, then do that submarine dive thing and simply unload the T Copter and end the day.
3 - After you lose go back to Design Maps and load the previously created map. Now delete every unit, and try not to take your finger off the A button. If performed correctly Nell will pop out and tell you "The Orange Star Army has lost!" (the fact that you have chosen to deploy other armies units is completely irrelevant and will work nonetheless).
4 - Now, this is REALLY important. DO NOT press the select button EVER after deleting the units. Press A to access the menu and make your way to exit. The screen may look like there's something wrong but don't worry it won't crash unless you press the SELECT button.
5 - Despite the fact that the screen looks kind of wrong head for either campaign or war room. In here just do what you'd normally do in design maps to deploy or change/add terrain/HQs/etc. You'll have to find the best way to adapt to the screen but trust me, it's just a mather of time. Remember NEVER to press the SELECT button!
6 - After you make all the desired changes you have two options: you either save the game and switch off your GBA (thus ending the freaky screen, turning everything back to normal but with the changes you've made) or you continue to play like that (thus discarting the need to follow steps 2 to 5 and allowing you to progress through the campaign in a fastastic winning streak). For begginers I would suggest the first option, but every man is his own boss so you do whatever you want.
This is a very usefull cheat and if you get used to it you'll be able to advance in rank easily (depending, of course, of your rank in the level, which depends on how you choos to win the level) and unlock every character in the game, which becomes rather boring seeing as you have to go through campaing at least 3 times! So please try this cheat and tell me what you think of it. My last recomendations are: don't worry about the screen looking freaky, that's normal and, I can't stress this enough, DON'T PRESS THE SELECT BUTTON after step 4. Doing this will probably cause the game to crash and freeze, although, as far as I know, as absolutely no side effects (no save file corruption, no errors, no nothing!)
Olaf's Sea Strike Mission: On Mission 4 use Andy then in Mission 6 defeat every enemy unit .
Rivals Mission: For the Battle Mystery, Captain Drake, Wings of Victory and Naval Clash missions use Sami. Defeat Kanbei Arrives mission in fewer than 8 attempts, Mighty Kanbei mission in 10 or less attempts and Kanbeiâs Error mission in not more than 12 turns.
Unlockables: Advanced Campaign When you have finished the main Campaign and have brought Advance Campaign you can play in Advanced mode by holding down the Select button before entering Campaign mode. You will encounter enemy opposition which are harder to defeat and you will receive more War coins when the mission is completed.
Open Up New Gameplay modes: Complete the final mission in Field Training to open up the Campaign, War Room, Design Maps and Stats options on the Main menu.
Unlock CO Drake: During Campaign mode Andy must be selected for Naval Clash, Wings of Victory, Captain Drake and Battle Mystery missions. When the Campaign mode is over Drake is availabe at the Battle Maps Section for 50 War Coins.
Unlock CO Eagle: Select Sami to win Captain Drake, Naval Clash, Battle Mystery and Wings of Victory missions before winning the Rivals battle. Eagle will the be available to buy in the Battle Maps section for 80 War Coins.
Unlock CO Grit: Complete Mission 4 in Campaign mode and select Max to complete it to be able to buy Grit for 50 War Coins. Grit will then be available to buy in the Battle Maps section for 50 War Coins.
Unlock CO Kanbei: Buy Kanbei for 50 War Coins in the Battle Maps section once you have completed the Campain mode in the normal difficulty setting.
Unlock CO Sonja: Buy Sonja for 60 War Coins in the Battle Maps section when you defeat Kanbei Arrives mission in fewer than 9 attempts, Mighty Kanbei mission in 11 or less attempts and Kanbeiâs Error mission in not more than 13 turns which unlocks the bonus Sonja missions which you also have to beat.
Unlock CO Sturm: To unlock Sturm you need to have purchased Sonja, Eagle, Kenbei, Drake and Grit from the Battle Maps section and then Sturm will be available to purchase for 100 War Coins but he can only be used in the Vs mode. Obtain Sturm quicker by using Max in as many battles as possible.
Unlock CO Nell: Buy Nell in the Battle Maps section for 80 War Coins once you have obtained an S-Rank in Advance Campaign and War modes.
Special Intel: Successfully complete all the Field Training missions to unlock the Field Training mission Special Intel which you will be able to in the list back at Field Training missions.
Use other CO's in multiplayer: Beat Campaign mode and then purchase other COâs in the Battle Maps section to use them in Multi-player.
Easter Eggs Gold Menu Icons Complete the condition listed next to certain icons in the Main menu to change them to a flashing gold from their original colour.
Stats: Regardless of rank complete all 20 War Room maps.
Rank: Obtain Rank 1as the final rank.
Records: Obtain gold Rank and Stats and Records will turn gold also.
Field Training: In all 14 Training missions obtain an A-Rank.
Campaign: Regardless of rank complete the Advance Campaign.
War Room: In all 20 War Room maps get an S-Rank.
Battle Maps: Purchase all the maps and unlock and buy every CO.
The following cannot be turned to gold: Vs mode, Design Maps, and Multiplayer menu.
Deleted saved games: When you turn on the Game Boy Avance Hold L + Right + Select.
Stop enemy from building: Position a medium tank or other troop type on an enemy building.
Load more units to transporter ships: Load a unit inside an APC then load the APC to a Lander or load a unit to a T-Copter and load the T-Copter to a Cruiser.
Ghost mode: To achieve Ghost mode you Hold B during your turn.
In-game reset: During game play press Start + Select + B + A. If the Game Boy Advance is linked to a GameCube doing this may cause data loss.