Join or create a match for either public or private (if you have friends) via the Network Game option. You only need to complete ONE level and it can be done with ANY character. Quickest way would be using a maxed character on Average Joe difficulty, selecting World 1-1 (Frozen Suburbs) and play through until the end of the stage. Alternatively you can select World 3 - The Main Stage - as this boss battle is super short on Average Joe with maxed stats.
Use This Thread to find people to play with!