Life bars can be done both in the gui scene or the game scene.
If you like more of a bar that is following the enemy (classical RTS style) You might want to prefer game scene (even if it’s possible in gui scene). It will give you a little less hassle when it comes to camera and positioning the bar correct. Then again using the gui scene you have better tools to create a bar.
We did one in our game a couple of weeks ago and then we decided to go with game scene. We created the bar in Spine and made an animation of the bar with 2 keys. This way you can control the animation in code using the spine cursor ( from 0 - 1). Doing it in the gui scene you have the possibilities to mask a bar and animate it that way instead.