How do I Spawn one of a sprite

So I'm making a game for a school project, and I wanted to make a boss that would spawn once you get to 50 points (You get 1 point for each enemy you kill) But when I make an action that states "System-Score = 50-System-create object-Boss on layer 0 at (0,0). I thought that this would work, but it keeps spawning the bosses constantly until the score is over 50 points. Is there any way to make it so that it would only spawn one of these?

"Trigger once" will work in this case.

Thanks for the quick responce, but I figured a way around it already, what you said probably would have been easier, but oh well. Thanks anyway.

Although, Actually, would you also happen to know how to make a game over screen appear when your character runs out of lives?

Although, Actually, would you also happen to know how to make a game over screen appear when your character runs out of lives?

Many ways to implement this but a good way would be to create an uppermost layer with black [or your choice of color] and during gameplay set opacity of the whole layer to 0.

When you meet your Game Over condition just fade the whole layer in or instantly set the opacity to 100.

Ok, Thank you guys, it really helped