Most enemies won't give you too much trouble alone, but together they can cause quite a panic. Occasionally, huge enemies like the towering "Hurker" barrel through the front lines and try to get up close and personal with you. Focusing fire on them with your friends is a good way to get them out of the fight quickly. For the rest of the mobs, there are plenty of silly weapons -- this is Insomniac, after all, developer of Ratchet & Clank -- that you can use to clean house. One fires propane tanks, to give you an idea. When things were exploding and erupting all around, the game looked great and the frame rate didn't dip for a second.
Though you all have a common goal, you're still competing to earn the most points, and you earn them much like you do in the campaign -- by killing with style. The map I played was filled with rails to grind, objects to bounce from, and things to explode. It's acceptable to run around on the ground and treat Sunset Overdrive like most shooters, but it's way more interesting to rain down icy death from the rails above before jumping down and shattering your foes.