You could add an array keeping track of all actors currently in the trigger box. When an actor enters the box, add them to the array. When they leave again, remove them. Then, in reaction to your Fire event, you use a ForEach loop over the entire array and call ApplyDamage for each actor.
I’ve attached some screenshots to a similar question I just answered: How to reduce stamina when player enters a box? - Blueprint - Epic Developer Community Forums
You’ll need to make sure to also remove any killed actors from the array. Maybe you can use the DamageCauser variable to inform the trigger box if an actor is killed.