Like in the first Advance Wars, you can erase your save data by holding LRightSelect before turning the game on.
Lab Map Locations
To gain Neotanks , you have to capture a specific city in a specific map to get the lab map, and then beat a hidden mission. You have to do this in each country.
Flak Attack: City north of your HQ, within range of your units starting positions.
Secret Level: Test of Time
Toy Box: South-Western most Neutral city, AKA: the one on the weird island. Secret Level: Neotanks?
Show Stopper: South-western most neutral city. Secret Level: Sea of Hope
Sinking Feeling: Southernmost city on the western island Secret Level: Danger X9