So since you have the distance, you can compare it to your trigger distance every tick and if it exceeds the trigger distance spawn the boss actor.
Now you need to make sure the boss is only spawned once. One way to do that is to put a DoOnce node between the distance check and the boss spawning. Another way is to put a Gate node at the very start. It should start open and be closed after the boss is spawned. A gate node is useful to disable an entire section of code whenever you want. The gate node is best for performance since you don’t update the distance travelled once its no longer needed.