First of all, make sure you have at least one co-op partner for this, or use a 2nd controller in split-screen (thanks Merv130). Then from the main menu, go to Campaign -> Arcade.
Play Act 1, Prologue (Troubled Past) to completion, which should take about five minutes if you skip the cutscenes using the Back button.
Quit to the main menu and repeat the above steps another four times.