One way of doing this is to have the area-of-effect have an overlapping sphere.
In Actor Begin Overlap, add the pawn to a list of pawns to damage.
In Actor End Overlap, remove the pawn from the list of pawns to damage.
In Tick, or on some timer, apply damage to any pawn in the list.
When the list is empty, nothing gets damaged.
If you have many of these, then each sphere can be small.
Efficiency is alright, as long as you keep it to “dozens” rather than “thousands.”
If you really do need “thousands,” look into how you might be able to set it up with Niagara or GPU compute or something like that.
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