The idea of using Cordon of Arrows is creative. I wanted to make it work but unfortunately for the PCs, all the arrows missed due to dice rolls...
For detection of the invisible monster, I guess I did not follow the rules accurately/strictly - In this particular scenario I ruled that the PCs did not know the location of the invisible monster, unless
-the monster attacked, or
-the monster was injured by an attack, e.g shot by the arrow, or
-marked by some other means, e.g covered with glitter dust, having an arrow sticking out from it etc, or
-one PC managed to engage the monster in melee, thus enable others to roughly perceive its location
Once the location of the monster was roughly detected by the above means, PCs could attack it with disadvantage on that turn. The monster could of course disengage and move away.
If I have a grid map for combat, the PCs can randomly guess a square to attack. If they make a correct guess and the monster is there, they can attack with disadvantage as well.
Just my way of running the scenario.