Health bar system is not working on multiple enemies on the scene.

Each 'Slider' as with each variable needs its own instance. So if your running so 100 enemies at a time, you will need it to track 100 sets of the variables and 100 sliders, one for each of them. You can use for example a Array to store them all then use a for each loop on it to update each one at a time. Also running it in the Update loop might be a issue with performance depending on just how many it needs to look at and update threw. You made a class for it, so just create a instance array of that class elsewhere to manage it as needed, then 1 update loop to show sliders. Create an array with the new command for how ever large you plan it on being, say public class enemyHealthManager { .... your class stuff }

public enemyHealthManager[] myEnemyHealth = new enemyHealthManager[100];