Hey there, you want to use GUIs to show the health bars above each enemy, right? Then since you seem to know what you're doing, here's a couple of things to hopefully get you up and running. Do ask if you need more detailled info.
I've found some stuff on pixelInset but I can find no reference to WorldToViewportPoint in any playmaker resources so its probably safe to say I'm trying to recreate the technique using the wrong methods, I hope someone can set me straight about this.
"screen to world space" - "world to screen space"
So you can use the world to screen space for the X/Y coordinates of your GUI. Then the offset could be the position of the target + a certain value on the Y axis, and then all that in screen space.
So in the same state as the GUI, get both the camera's position each frame, and the target position each frame. Add a "Is Visible" action to make sure only the visible stuff is called each frame. That Is Visible action could be called every second or so to save performance.
Then use get Distance to get the distance between the camera and target to use in a float operator to rescale the size (farther away = smaller gui).
That should be all.
Like I said, if you need more details, say so, I'm somewhat in a hurry right now, but this is a nice topic.
But do tell, why no mesh bars?! They would be the best choice for this both performance and effort wise. All you'd need is to use a look toward action to make them always face the camera.