Ok, I made these rules up off the top of my head, so if some of the numbers need tweaking, be my guest.

These are rules for one person playing the Encounter Deck (EP) against any number of Hero Players (HP). HP rules will remain the same except where noted. The EP only wins if he knocks out every HP either through threat or KO.

The EP's hand size will be base of 4 increasing by 2 for each additional HP. So 1=4, 2=6, 3=8, and 4=10.

Resource phase - The EP will draw a number of cards equal to his hand size.

Planning phase - EP has no action here.

Quest phase - EP player chooses 1 card per HP from his hand and places it in the staging area after all heroes have committed. (This offers choice of no-threat treachery cards, nasty enemies for engaging or load up on threat with locations)

Travel phase - EP has no action here.

Encounter phase - As normal, except in case of ties, EP chooses which will engage. Also, the optional engagement could be removed if it proves too easy. In fact, it might have to be removed, haven't tested it yet.

Combat phase - Monsters still attack first, but EP chooses which monster will get which shadow card from his hand. EP must deal each engaged enemy a shadow card and if he runs out of cards in his hand, too bad. Defenders are still chosen by the HP.

Refresh phase - EP discards any remaining cards in his hand.

I think this gives enough interactivity to the encounter player, without changing any fundamentals. I expect some tough choices between staging a card or using it as a shadow effect and such. Even so, I think I'd choose to be the EP in most cases.