Make Your Platform Game a Shooter

Add this to the Platform Game you already have.

Disclaimer:  This is a simple game that does not encourage violence. 

Create a Bullet Sprite and Monster Sprite

Create a sprite that looks like a bullet. 
Create a sprite that looks like a monster. 

Create an Object Monster and Object Bullet

Create an object for the bullet named obj bullet
Create an object for the monster named obj monster.
Make your Object Person Create a Bullet
  1. Go to your object person. 
  2. Add a Key Press event for the spacebar. 
  3. Add a Create action. 
  4. For the Create action add these numbers. 
Make the object bullet move and destroy after leaving the room.
  1. Go to your object bullet. 
  2. Add a create event. 
  3. Drag the Horizontal Speed Action over. 
  4. Set the following numbers. 
  5. Add an "other" event. 
  6. Drag the destroy action over. 
Make the monsters move in your game.

I set up my room like this.  The monsters will go back and forth between the platforms.
Working with Object Monster
  1. Open up Object Monster. 
  2. Add a create event. 
  3. Drag the 8 pointed direction action over. 
  4. Set the buttons like this. 
  5. Add a collision event with the platform. 
  6. Drag the reverse vertical direction over. 
  7. Add another event for collision with the bullet. 
  8. Drag the destroy action over. 
  9. You may want to add some point when the bullet collides with the monster.  If you do, drag the action over. 
Make your monster explode.