There are two simple nodes.
“Any damage” and “Apply damage”.
Any damage will receive all types of damage (unreal offers a few out of the box) and Apply damage will trigger that event on the other actor.
Now to fire this event you simply need some kind of hitbox and once it does collide or overlap with something, take the resulting “Other Actor” and pull a line from that pin to your “Apply Damage” node.
Any Damage will then receive an input and you can handle your HP in the actor which just got hit.
So Apply damage in your bullet or inside of your spikes blueprint. Any Damage in your Character or whatever should get hit.