they could easyly do something like: 1/("number of Players involved" +1)=damage the Monster receives.

with that the Monsters would scale automatically to the numbers of Players and having more Players would be a plus: example
1 Player =50%dmg
2 Players = 33% for everyone (66% all togehter if they have the smae Standard dps)
3Players = 25% per PLayer (accumulates to 75%)
..
..
..
100 Players = ca. 0,99% per Player (ca 99% dmg accumulated)

With that the Monsters would scale and the difference between 1, 10 or 100 Players wouldn't be so big.