This is such a basic question that it is a bit embarrassing to raise, but the rules and examples in the book really failed to address this in any meaningful way.
You have a large, powerful, high defense Earth Elemental. It is the only opponent. The party attacks it using hack & slash and volley.
The first player attacks, rolls an 8. He does damage, the Earth Elemental does its damage back.
The second player attacks…. what now?
As I see it there are three possibilities.
a) The Earth Elemental moves at hyper-sonic speed, an enemies speed is always increased by a factor equal to the number of opponents.
b) Because the Earth Elemental is smashing player 1, it is now helpless against all other players who needn’t even roll to do their damage.
c) Just like the rule for multiple monsters, regardless of how much or how little damage PCs would normally do, they just add a +1 bonus for damage to the initial player’s damage roll.
d) You aren’t supposed resolve the first action until every player has chosen their move. Only one player actually rolls Hack & Slash, everyone else uses Aid Another. And regardless of how many other players aid, the actual player taking the risk of being damaged only gets a +1.
A related question– what do you do when you have 7 monsters and 5 PCs? Do you divide the monsters up between the PCs and decide which are trying to do damage to which PCs and the PCs can only attack their own opponents? Do the PCs take the risk of being damaged by whichever opponent they attack and the opponent always gets a +6 bonus?
If the enemies are 3 Kobolds, 2 Orcs and an Ogre, does this mean that the Orcs and Ogre do nothing more than having just another Kobold so long as the PCs only attack the Kobolds?
None of these options seem particularly satisfactory. It would also be helpful if someone could point out the page number that has the definitive answer as to how this is supposed to work. It strikes me that every example in this book was written with the mindset that this game is only meant to be played with 1 GM and 1 PC (or possibly 2 PCs, but that’s a stretch).