There's not a lot to say about this one. You can get this by playing an online or local match with two additional local controllers but it's probably easier to boost. Quite simply, start a local game (by choosing "Bot Match"). Make sure to have three controllers and sign in your two extra controllers as guests. Before starting the game, use the bumpers to set the AI difficulty to 0/5 which will make the computer enemies incredibly simple. Choose a character that you're comfortable with and push forward playing the game as normal.
It can also be handy to position one of your alternate characters behind one of your turret lines just so you can quickly defend by switching to the other controller. Or just have two friends come over and help.