==== UPDATE 1 ====

After so many tests, this is what I found.

There is 2 cases.

► 1st case : Single attack ( A attack M )
This is really and only based on level of attacker and monster.

► 2nd case : Multi attack ( A, B, C attack M )
In other words, this is combo attack.
it is different. Because there will be extra damage.
The extra damage is the cooperative damage.
I found this after my test :

M (monster) has 70 HP
After only A attacked, M has 40/70 HP, so A did 30 damage.
And after only B attacked, M has 35/70 HP, so A did 35 damage.
So it's easy to see that A & B can do 65 damage (30+35). And M will have 5/70 HP.
That means M will not die.
It's the theory. But the truth is different.
Because we forgot the extra damage.
I tested. And M dies. So where is the 5 more damage ?
This 5 more damage is the extra combo damage.
It happens when multi attackers combo.
Now I'm sure that "extra combo damage" exists.

Moreover, "extra combo damage" also gives you "extra exp".
And if A, B & C attack M,
I doubt that the highest SPD among A, B & C will take the "extra exp".
That's why SPD affact our EXP gaining.
But it's only doubt. I need more time.
And I will tell about this in the next update.