A little bit of experience, and soon you'll find Closing the Portal to be not so hard. (For example, Oni evolution, adding one card to the Ork, means that, without fail, the Ork can kill almost any one obstacle immediately--or, if others have drawn well, finish off two obstacles in the first turn. An extra hit point for the Elf is an amazing thing. Add a few more initial bumps, and its a piece of cake.) I'd say the GenCon ratio of success had a lot to do with player inexperience. As we've played, we've gotten a lot better at making effective use of assist cards and using the right cards for the right jobs. Monofilament whips are your friend, as are any cards that simply do levels of damage.
I like that Crossfire is challenging. Many cooperative games are too easy., We really have enjoyed Sentinels of the Multiverse but we've had to go to random selection and advanced difficulty because it was really no challenge. The novelty of having a cooperative game actually present an occasional challenge is wonderful.
For the 4+ player rules, when we do run with extra players so far we've just kept adding a Crossifre card to the difficulty level, and giving everyone a minimum of one obstacle, but no increased rewards. Still, I'm worried this is still making it too easy.