task Glow {
   loop(4){
      Glowtrail();
      wait(53);
   }
}

task Glowtrail(){
   let randir = rand(-0.5,0.5);
   let obj = CreateShotB2(BossX,BossY-45,randir,-3,0,0.05,0,2,308,15);

   while(!Obj_IsDeleted(obj)){ 
     TrailEffect(20); // I assume you want 20 frames before it's deleted.
     wait(10);
   }

   task TrailEffect(delay)
   {
      let LgtX = ObjMove_GetX(obj);
      let LgtY = ObjMove_GetY(obj);
      let MiniLgt = CreateShotA2(LgtX,LgtY,0,rand(0,360),0.001,0.1,74,11);
      wait(delay); // it's 20 in this case
      ObjShot_FadeDelete(MiniLgt);
   }

}