One of my players is asking if he can use Wall of Fire to block off the line of sight of enemy archers. The spell description says the wall is "opaque", so this seems like a valid thing to try.
However, because this opaque wall would make both sides unseen from each other, the advantage and disadvantage would cancel out, and it would provide no immediate tactical benefit.
When you attack a target that you can’t see, you have disadvantage on the attack roll.
When a creature can’t see you, you have advantage on attack rolls against it. (PHB p. 193-4)
In the interest of trying to see if he can get any tactical benefits out of this – if he blocked the line of sight of the enemy archers in a group but not that group's melee fighters, would he/others be able to take the Hide action from the archers even if the melee fighters could still see him?
If he can Hide in this way, that would mean the archers would no longer be able to know his precise position and would need to guess his square before making an attack.
Best Answer
You can hide from the archers separately to the melee fighters
From PHB pg. 177:
When you hide, you make a stealth roll. This is against the passive perception of the enemies you are hiding from. If some have higher passive perception than others, and your stealth roll happens to land in between, you will be hidden from those with lower passive perception but not hidden from those with higher passive perception.
So, this sets the precedent for being able to hide from some but not all enemies. If the archers cannot see you, you can hide from them, but the melee fighters can see you and thus you cannot hide from them. The fact that the melee fighters can see you does not stop you from attempting to hide from those who cannot see you.