I was really intrigued by this. I decided to keep trying to figure it out. This is far from done and may contain errors but the concepts are there. Click as fast as you want to fire once per tower. The tower will target the weakest. It should probably target the closest but I decided to target the weakest and they target in the same order.
Click to fire once from each tower. The enemies are draggable for debugging.
Project File
doug13579/Gdevelop-Tower-defense-example (github.com)