What is the gaurdian plane and how to get it?

There is no plane called "Guardian" in AMS.

Here are the planes and how to acquire each of them:

Vampire - You get this when you start the game

F117 - You can purchase this many different ways. Happy purchase(on plane month), Events, and in game purchase

S-Wing - Get parts from tan army event, and in game purchase.

Storm Wind - Air force Coins

Pegasus - Events, and in game purchase.

Conqueror - Events, and in game purchase.

Ice Dragon - In game purchase.

Spaceship - Event, and in game purchase(only available at certain times of the year).

Flame Feather - Event from in game purchase.