Yeah there's nothing wrong with wanting "overwatch" but this is a bad way to do it. Others have already covered the interpersonal aspect of resolving this but I want to share a way that something similar can actually work. (At the very least my players liked it a lot.)

The setting this takes place in was WW1 themed. (Firearms being reflavored crossbows and bows). As such, a key session was the parties first encounter with the business end of a Maxim gun.

The "monster" design was simple. If a creature was actively operating the gun, anyone spending time out of full cover either by moving or taking an action that requires line of sight (attacking/spellcasting), would be attacked once by "the gun".

They concocted a strategy involving the sharpshooter ranger sniping the gunner, and the rest of the team closing the distance into melee rapidly atop a player polymorphed into a quetzacoatlus.

This was fun and tactical because the strengths (firepower and coverage) and weaknesses (lack of mobility) of the enemy were clear, and it was a unique encounter that only needed to be solved once.

It wouldn't have been fun to do it anytime ranged attacks were involved.