After posting the rough draft of the rules, it might not be easily apparent how Planar Conflict is cooperative. Here are some ways in which players can work together to defeat Infernals:
1. Card trading. Need a 3 point creature to get your Shield up to 30? Didn't draw a Shield? If one of the other players has what you need, make a trade. Just remember that you can only trade cards in your Storage Hand during the Draw and Organize Phase.
2. Run block. You can keep Vassals busy while your partner makes a run at the Infernal.
3. Save your friend's butt. Is your buddy's Guardian or depleted Shield about to get pummeled by a Vassal? Move your Shield over to intercept it. Your friend will owe you one.
4. Sacrifice. Put some damage on an Infernal so the next Shield can finish it off.
5. Strategize before playing. You know what Infernal you've chosen to face. Have your friend help you build your challenge deck and vice-versa. He may think of something that you didn't.
6. Combine Dispel Magics and Bribery Cards. Runes and Artifacts take 3 Dispel Magic spells to dispel them. Some Vassals require 3 bribery cards to bribe. If you've got 2 cards and your friend has 1, you can Dispel or bribe problems away together by combining them.
I hope now you have a good grasp of how players can work together to take down an Infernal...