You may want to try get all actors by class and then, under the class option, select third person character. This will result in an array, do a for each loop out of the array so that you can cycle through the characters applying damage to all third person characters in this instance. You could limit damage to characters in a certain range or in line of site etc.
I make an “create player” in the Level blueprint to get an splitscreen. I drag both players out into the level. But the AI just runs to Player1 even when the AI just see’s player2. It’s like the player1 is the magnet for the AI, even tho he see’s player2.
Here’s a blueprint I made previously that does damage to all pawns within 1000 distance of the sphere mesh I had in the level. It’s basically a pulsing orb and if things get to close it applies damage. I have a healing orb that heals, I use the same object just put in a negative damage (-5 for example). I’m just learning but hopefully this gives you an idea.