Storm Trooper Achievement in Rocket League

Very Rare items will be viewable from the Manage Inventory section. You can use the filter to find them or just look for items labeled with a purple font. There are several ways to get a very rare. They can fall as a random drop after an online game, you can get them by opening a crate or you can trade in five rare items for one.

Once you have one and it is equipped, head into exhibition and make sure the arena you choose says STORMY. Now play a game. You can set the goal limit to one in mutator settings to end it quick.