Basic problem is that the condition "numEnemies = 20" is true on every tick, and boss is spawned every tick as long as numEnemies is 20. Many ways to solve this, and you can use any of these you like:
- Add "Trigger once" condition to your event after numEnemies = 20 condition.
- Add 1 to numEnemies after spawning the boss (then numEnemies is 21 and not 20).
- Add new global variable "BossSpawned" with initial value 0. Add condition BossSpawned = 0 to your event. Set BossSpawned to 1 after spawning.
- You can add second condition using "count" expression so that Boss is not spawned is there is more than 0 instances of boss. (I haven't used "count". So I'm not sure about this.)