A giant IRL (In Real Life) shooting game in an abandoned warehouse is targeted by a masked serial killer who starts to pick off the warring combatants under the cover of darkness.

This should be defined as an augmented reality game instead of an In Real Life shooting game.

IRL shooting games are neither paintball nor laser tag. The one I saw had players walking through a warehouse decorated to look like a battle zone. Players wore augmented reality visors and they shot at enemies that appeared in their visors using game controllers shaped like assault rifles. In that scenario, everyone wore headsets and was in constant communication with everyone else and the warehouse was under continual surveillance by the game controller who gave the players hints when they got stuck.? Hard to imagine how an intruder could pick the players off one by one without the others or the game controller noticing.

Maybe you should change the premise such that the game control turns rouge and has hidden death traps in amongst the props.

It is always better to have your lead character in the logline.