How do I make the following progression system?

How do I make enemies increase their health depending on the player's level?

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example:

Player level: 1 --- enemy hp = 10

Player level: 2 --- enemy hp = 20

Player level: 3 --- enemy hp = 30

etc

This was the event I did

Each enemy will need an instance variable representing health. When the enemy is spawned, you set its instance variable according to the player's level. Or, if you are increasing the health of existing enemies, you can Add To that instance variable for all enemy instances (if no instance is specified, it should affect all instances).

I forgot to mention it, but the enemy already has a variable called HP and starts with 10 health.

..how should I start the event?

You should add it to line 12, the event that handles leveling up.

Increases the HP of one enemy but not all subsequent ones that appear :C

How are the enemies spawned?

When the enemy is spawned, you set its instance variable according to the player's level.

Yes. Add another action to that event, with "enemy" as the object, to set the value of the instance variable.