How do i make a gaster blaster system? (from undertale)

Something like this

Generally speaking, beams are constructed out of three sprite object parts, a stretchable or tile able center part, and two caps at the beginning and end. Depending on the look of your beam, the caps can be the same object, mirrored, or you can possibly do without one or the other depending on what is visible on screen.

Normally you would put the pieces in a container to automatically create and destroy each other. You can use imagepoints on the center object to position the two caps. To pulse the width, I recommend using the sine behavior.

Edit: Or you can use the 9patch object, since it is apparently now rotatable (when did that happen?). It should work nicely.

hm i see, i will give it a try. I kinda need to figure out how to make the sprite go to the mouse position xd

On created set angle towards position (mouse.x, mouse.y).

You should try the beginner tutorials.

construct.net/en/tutorials/beginners-guide-construct-1