There was some weird bracket stuff going on.
I rewrote and cleaned up the code a bit, now you should also be able to use it on many enemies:
Code:

---

define sword_base = skill12; // using skill12, 13, 14 to store the sword_base coords
define sword_tip = skill15; // using skill15, 16, 17 to store the sword_tip coords
define healthpoints = skill18; // just another name for skill18


action enemy_fight
{
	var move_dist[3];
	var distance;
	
	my.healthpoints = 100;
	
	c_setminmax(my);
	wait(1);
	
	while(plBiped01_entity == null)	{ wait(1); }
	
	while(my.healthpoints > 0)
	{
		distance = vec_dist(my.x, plBiped01_entity.x);
		
		if(distance < 200)	// the player approaches the enemy
		{
			vec_set(temp, plBiped01_entity.x);
			vec_sub(temp, my.x);
			vec_to_angle(my.pan, temp);
			my.tilt = 0;
			
			move_dist.x = 5 * time_step;
			move_dist.y = 0;
			move_dist.z = 0;
			c_move(my,move_dist,nullvector,IGNORE_PASSABLE);
			
			ent_animate(my, "walk", my.skill19, ANM_CYCLE);		// play walk frames animation
			my.skill19 = cycle(my.skill19+5*time_step,0,101);	// "walk" animation speed + loop animation
			
			if(distance < 50)		// if the player comes closer than 50 quants attack him
			{
				my.skill20 = 0; // reset "attack" frames
				
				while(my.skill20 < 100)
				{
					vec_for_vertex(my.sword_base,my,306);
					vec_for_vertex(my.sword_tip,my,291);
					
					c_trace(my.sword_base,my.sword_tip,IGNORE_ME|IGNORE_PASSABLE);
					
					if(you)
					{
						you.healthpoints -= 4 * time_step;
					}
					
					ent_animate(my,"attack",my.skill20,ANM_CYCLE);	// play attack frames animation
					my.skill20 += 5 * time_step;	// "attack" animation speed
					
					wait(1);
				}
				
				wait(6);
			}
		}
		else	// the player is farther than 200 quants away
		{
			ent_animate(my,"stand",my.skill21,ANM_CYCLE);		// play stand frames animation
			my.skill21 = cycle(my.skill21+2*time_step,0,101);	// "stand" animation speed + loop animation
		}
		
		wait(1);
	}
	
	// enemy was killed:
	while(my.skill22 < 80)
	{
		ent_animate(me,"death", my.skill22, ANM_CYCLE); // play death frames animation
		my.skill22 += 1 * time_step; // "death" animation speed
		
		wait(1);
	}
	
	my.passable = on; // the corpse can't be hit by the sword from now on
}

---

But be aware: This code does not include that the enemy takes damage (reacts to a certain event), thus your weapon code needs to handle this.
I'm not sure how the templates work though.