There's several ways to get this done, one of which being your good enough with a character of your choice and your lucky enough to not have the computer being a pain against you. Here's the way I found the easiest. Set the difficulty as low as possible, and select Leona. As soon as you start the match, jump forward and instantly hold  the second Leona leaves the ground. Press  in the air near the opponents head followed by  again when you land, while still holding . As the crouching  move strikes, press  and  to do her slasher move. This 3 hit combo does a lot of damage, and if repeated throughout the entire match you can finish the first team in about 18 seconds. My best record using this method is 3:01 meaning there's plenty of room for mistakes and such. This isn't too hard when you get used to it, so go into training mode to practice the combo if nessecary.