Well, first add a true global boolean called 'Can_Spawn.' Then add a condition to your enemy spawn logic: Can_Spawn is true. This will make it so enemies can only spawn when the boolean is true.
For the next thing, add a false boolean called 'Boss_Spawned.' Then add an event:
-> Score => 500 (or whatever score you need to reach in order for the boss to spawn)
& Boss_Spawned is true(invert this condition)
--> Set 'Boss_Spawned' to true
--> Set 'Can_Spawn' to false
--> Enemy -> destroy (This will pick all enemies if you have not referenced any in this event)
--> System -> Spawn another object -> Boss
If this does not make sense, you may want to read this: construct.net/en/make-games/manuals/construct-3/project-primitives/events/how-events-work
Hopefully this helps,
Give me a Pixel