" i have been trying this and it does not work"
Often, it is useful to show the failed attempt, so we can show you what was wrong...

okay then

Notes:
- Try Edit > Auto Format (or Ctrl+T), it will help you!
- Booleans don't need to be tested with ==
- You assign -1 to healthbar, then you test if it is zero, it is unlikely to happen...
- You probably have a syntax error on the last test, since you assign healtbar to zero, instead of testing it.
A possible fix:

1. class Enemy {
2.   float posX;
3.   boolean kill;
4.   int typeScore;
5.   int healthbar = 2;
6. 
   
7.   Enemy() {
8.     posX = width+(int)random(500, 1000);
9.     kill = false;
10.   }
11. }
12.  
13. class Wretch extends Enemy
14. {
15.   Wretch() {
16.     super();
17.     typeScore = 3;
18.   }
19.   void display() {
20.     if (!kill) {
21.       image(wretch, posX, 375);
22.       posX -= 1;
23.     }
24.     if (posX < 55)
25.       state = 4;
26.     if (attack)
27.     {
28.       healthbar -= 1;
29.     }
30.     if (healthbar == 0)
31.     {
32.       kill = true;
33.     }
34.   }
35. }

-= means "alter the content of the variable on the left, subtracting the value on the right".