Dungeon Explorer 2 is what we want to play co-operatively
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DE2 is so dang awesome. I haven't had a chance to investigate it, but I'm fairly sure they increase the boss HP for multiple players, making the boss fights incredibly tough, the more players you add. The other issue is that if you have 5 players, and only 1 survives the boss fight, only one gets the level-up, meaning you are going to have lots of lagging behind in levels. Finally, certain bosses (Damn Tree Dragon!) are a pain in the ass for multiple players - there isn't enough room for everyone to move to escape his attack!
It may sound like I'm negative towards multiplayer, but in truth I love it. However, expect the difficult to ramp up.