Well, the solution depends on your current setup and design.
So I believe that you can use the first part of your idea is good and simple
-If Line Trace detection = True, then Null Damage = On
-Tag Check
Link: https://www.youtube.com/watch?v=_SjhBlj4Mu8
-Does Implement Interface
Link: https://www.youtube.com/watch?v=uz-XI8VPXgk
-Cast to a specific actor
Link: https://www.youtube.com/watch?v=HG9oxPAvowQ
But the second part needs some more iteration.
I have two suggestions:
1.Save the Enemy actor if the linetrace hit -> “Next frame” -> Check if Linetrace hit anything or Linetrace hit other actor -> Set Trace detection = False
2. Set a function on the Enemy actor which disable its collider for a specific amount of time. Everytime the Linetrace hit the actor, it retrigger the time, simple solution could be “retriggerable delay”