How to decrease a Slider value by time?

@radenzilu Did this work for you?

@Mavina First of all, thank you for your concern. I tried and it didn't work. Maybe, it's because I am a newbie. LOL Anyway, your code open my eyes for a lot of things here. I'll paste my code here so you can see what I did wrong.

 using UnityEngine;
 using UnityEngine.UI;
 using System.Collections;
 
 public class EnemyHealth : MonoBehaviour
 {
 public int startingHealth = 100;
 public int currentHealth;
 public Slider shootSlider;
 public Image Fill;
 public int powerCharge = 10;
 public int powerReset = 20;
 public float sinkSpeed = 2.5f;
 public int scoreValue = 10;
 public AudioClip deathClip;
 Animator anim;
 AudioSource enemyAudio;
 ParticleSystem hitParticles;
 CapsuleCollider capsuleCollider;
 bool isDead;
 bool isSinking;
 
 void Awake ()
 {
         anim = GetComponent <Animator> ();
         enemyAudio = GetComponent <AudioSource> ();
         hitParticles = GetComponentInChildren <ParticleSystem> ();
         capsuleCollider = GetComponent <CapsuleCollider> ();
 
         currentHealth = startingHealth;
     }
 
 
     void Update ()
     {
         if(isSinking)
         {
             transform.Translate (-Vector3.up * sinkSpeed * Time.deltaTime);
         }
         UpdateHealthBar ();
 
     }
 
 
     public void TakeDamage (int amount, Vector3 hitPoint)
     {
         if(isDead)
             return;
 
         enemyAudio.Play ();
 
         currentHealth -= amount;
                     
         hitParticles.transform.position = hitPoint;
         hitParticles.Play();
 
         if (currentHealth <= 0) {
             Death ();
         }
 
     }
 
     void Death ()
     {
         isDead = true;
 
         capsuleCollider.isTrigger = true;
 
         anim.SetTrigger ("Dead");
 
         enemyAudio.clip = deathClip;
         enemyAudio.Play ();
 
         shootSlider.value += powerCharge;
     }
 
     public void UpdateHealthBar() {
         if (shootSlider.value >= 100) {
             Fill.color = Color.cyan;
         }
     }
 
     public void DecreseEnemySlider()
     {
         StartCoroutine(DecreseSlider(shootSlider));
     }
 
     IEnumerator DecreseSlider(Slider slider)
     {
         if (slider != null)
         {
             float timeSlice = (slider.value / 10);
             while (slider.value >= 0)
             {
                 slider.value -= timeSlice;
                 yield return new WaitForSeconds(1);
                 if (slider.value <= 0)
                     break;
             }
         }
         yield return null;
     }
     public void StartSinking ()
     {
         GetComponent <NavMeshAgent> ().enabled = false;
         GetComponent <Rigidbody> ().isKinematic = true;
         isSinking = true;
         ScoreManager.score += scoreValue;
         Destroy (gameObject, 2f);
     }
 }

Thanks Mavina, really thanks!

Mavina, your code and my code are both working fine and decreasing the slider. The problem is that it doesn't go from 100 to 0... It stops before reach 0 when is decreasing. The weird thing is when I press play, it works diffferent every time. Sometimes it decreases until the half of slide, sometimes just a little slice and sometimes go near to 0, but doesn't reach. Do you know what is happening? Thanks anyway!