How do you resolve evade & / or combat skill checks when you arefaced with more than one monster? The AH basic rules just says I must fight or evade each monster. If I try to evade, do the monsters awareness numbers combine, or must I resolve each monster separately? If I resolve each individually but fail one, then does the ensuing battle cause the other one to join in? Same with combat: Does the investigator fight them one at a time or do monsters combine their numbers. I know some players would do either according to their own improvised rules, but what are the official rules on this? Thanks!

You battle/evade each monster one at a time.After defeating one monster you proceed to evading/ combat with the next(your choice). You have to deal with all the monsters in your current location on the same turn. The monsters don't join forces or gang up on you. Each combat/evasion is resolved individually.

If you feel that the odds are against you after combat with a monster has already begun(having lost too much stamina in previous rounds, for instance), you can still try to evade it, unless the monster in question has Ambush. Monsters with this ability can only be evaded before combat with them begins.

Thanks! I thought as much but its good to be sure about it.

Come to think of it, monsters that sometimes gang up would be an interesting variation or house rule. It would force other players to combine their forces too. Maybe when their are monsters that have some other common factor. Perhaps when evation result is poor or horror check is well below the difficulty factor. Since creatures like that would smell the fear giving them the pack animal instinct.

I think a similar variant was proposed on boardgamegeek.com

You could houserule that any monsters in the same area with identical dimensional symbols join forces, combining their toughness and stacking stamina damage and combat rating. This can be a cool idea for a scenario.

I generally don't houserule Arkham Horror because I think the game is well-balanced and challenging already-just add some expansions and you'll see- it becomes much harder.