action monster1
{
var goback = 1;
my.enable_scan = on;
my.event = monster_1;

while (goback > 0)
{
my.x += 1.5;
ent_animate(my,"walk",my.skill46,ANM_CYCLE);
my.skill46 += 4 * time;
my.skill1 += 1 *time_step;

if(my.skill1 > 12)//adjust as required
{
my.x -= 1.5;
my.pan = 180;
ent_animate(my,"walk",my.skill46,ANM_CYCLE);
my.skill46 += 4 * time; 
}
return;
}
}

var counter = 0;

action monster1
{
     my.enable_scan = on;
     my.event = monster_1;
     while (1){ wait(1);

          //always loop animation in a while
          ent_cycle("walk", my.skill46);
          my.skill46 += 4 * time;

          if(my.x < 12){//use a multiple of 1.5 (3, 4.5, 6, 7.5...12)
               my.x += 1.5;
               counter += 1.5;//same as your movement amount
               my.pan = 0;// you need to reset the pan here
          }

          if(my.x >= 12){
               my.x -= 1.5;
               counter += 1.5;//keep adding, removes further coding
               my.pan = 180;
          }
          if(counter> 24){counter = 0;}//reset the counter, cause the enemy should be back where he was.
     }
}