Finally after reading the FAQ, ES is almost basically like Omens except for a few things. The most important, thematically and difficulty wise, is that you don't get to decide where the monsters go. It keeps you guessing. Should we take out this adventure now or chance a monster summoning. If there are four monster tasks out, then you know you can wait a couple turns before you start to panic. However if you don't know where the monster will be summoned, then it creates a lot of tension.
Anyways, the simple variant I use is to distribute the monsters randomly. If there are mulitple monster tasks out then you roll randomly amongst them. If all monster tasks have been filled then the monster can go to any adventure on the board. ( A die 6 should be sufficient for most games, although I keep my d8 or even d10 handy just in case)
For less chaotic games you could follow the one monster per adventure rule and randomly place them to remaining adventure cards without monsters. It's not as "fun" in my opinion.
Anyways, try it out. I've played many times this way and the difficulty of the game is always intersting.