This trophy is titled "Spoonful of Sugar" and it's for staying afloat while playing as Rachel for 25 seconds. While this might sound ridiculous, as it would be with any other character, it's nothing too demanding.
To start, make sure you either have a friend or second controller handy. With both controllers, make sure Rachel is the selected character. Upon the start of the match, do a super jump by pressing down and then up immediately after. You'll jump slightly higher than normal, follow this with another up and do a double jump. After you've reached the peak of the second jump, press upwards and X. After raising yourself up hold circle and continue to use the upwards and X another 3 times with your main controller, giving yourself some time between each one in order to make them last. You'll be allowed 4 with your main controller, then you must press upwards and X with the second controller to help keep you afloat. Use these sparingly as well and stay in the air as long as possible.
It should only take the 4 bars from each player in order to pop this trophy. It will be won shortly after you land if done correctly. This will be easier with a friend helping, however it is possible to do on your own with 2 controllers. Make sure to hold circle in order to keep your umbrella open.