Stop Monsters Spawning Add Boss?

Hello troops, wondering if anyone can help, I am trying to stop all 'monsters' spawning and spawn a big boss 'monster' after so many kills?

Is this possible?

I would imagine it would go something like,

<font color=red>

EVENT(1)

(<monster> on destroy X.amount)

ACTION(1)

(monster destroy)

(object add boss)

EVENT(2)

(<boss> on destroy 1)

ACTION(2)

(boss destroy)

(object add monster)</font>

Does anyone know how to make this possible as I am having trouble de-spawning all 'monsters' on X.amount killed and adding boss object. Then again the other way.

spawnBoss.capx (r139)

Basically - use a variable to track how many monsters are killed.

Monster: on destroyed -> add 1 to variable.

When the variable reaches some amount, destroy all monsters and spawn the boss.

Only spawn monsters if the boss isn't there (boss.count = 0)