Space Invaders monster movment? How?

function Mother::onLevelLoaded(%this, %scenegraph)
{
   $Mother = %this;
   echo(%this.getLinkPoint(2));
}

function Enemy12::onLevelLoaded(%this, %scenegraph)
{
   $Enemy12 = %this;
   moveMap.bindCmd(keyboard, "w", "Enemy12Up();", "Enemy12Stop();");
}


function Enemy12Up()
{
   $Enemy12.dismount();
   $Enemy12.setLinearVelocityY(20);
}

function Enemy12Stop()
{
   $Enemy12.moveto($Mother.getLinkPoint(2),10,true,true,true,0.1);
}

function Enemy12::onPositionTarget()
{
   $Enemy12.mount($Mother);
   $Mother.setLinearVelocityX(20);
}

function TestApp::addSprite( %this, %x, %y )
{
   %s = new t2dStaticSprite()
   { 
      scenegraph = %this.sg;      
      imageMap = "orcImageMap";  // CHANGE THIS TO YOUR OWN IMAGE MAP!!!
      frame = 0;  
   };
      
   %s.setPosition( %x, %y );
      
   %this.os.add( %s );
}

function TestApp::onAdd( %this )
{
   %this.sw = scenewindow2D;
   %this.sg = sceneWindow2D.getSceneGraph();
   
   %this.os = new t2dSceneObjectSet()
   {
      scenegraph = %this.sg;
   };
   
   %this.addSprite( 0, 0 );
   %this.addSprite( -20, 0 );
   %this.addSprite(  20, 0 );
   %this.addSprite( 0, -20 );
   %this.addSprite( 0, 20 );   
   
   %this.step = 0.25; // move unit per tick
         
   %this.camoutline = new t2dShapeVector();
   
   %this.camoutline.setFillMode( false );
   %this.camoutline.setLineColor( "1 0 1" );
   %this.camoutline.setPolyPrimitive( 4 );
   %cs = %this.sw.getCurrentCameraSize();
   %this.camoutline.setSize( getWord( %cs, 0 ), getWord( %cs, 1 ) );
   %this.camoutline.addToScene( %this.sg );
         
   // create a shape vector to show the outline of the object set
   %this.outline = new t2dShapeVector();
   
   %this.outline.setFillMode( false );
   %this.outline.setLineColor( "1 0 0" );
   %this.outline.setPolyPrimitive( 4 );
   %this.outline.setSize( %this.os.getWidth(), %this.os.getHeight() );
   %this.outline.addToScene( %this.sg );
   
   %this.setTimerOn( 32 );
}

function TestApp::onTimer( %this )
{         
   // We just custom check the object set has hit the sides of the screen and switch direction	
   %cs = %this.sw.getCurrentCameraSize(); // camera size
	%cw = getWord( %cs, 0 );               // camera width
	%rc = %cw * 0.5;                       // right screen limit
	%lc = -%rc;                            // left screen limit
	%ow = %this.os.getWidth();             // object set width
	%hw = %ow * 0.5;                       // half object set width
	%lx = %this.os.getPositionX() - %hw;   // left side of set
	%rx = %lx + %ow;                       // right side of set
		
	// Left or right side hit screen edges?
	if( ( %rx + %this.step > %rc ) || ( %lx + %this.step < %lc ) )
		%this.step = -%this.step; // yes so toggle direction
		
   // Next position
   %px = %this.os.getPositionX() + %this.step;
		
   // Set the object set position
   %this.os.setPositionX( %px );
   
   // Set the outline position so we can see where the set is
   %this.outline.setPositionX( %px );
}

// Create a custom scene graph
sceneWindow2D.setSceneGraph( new t2dSceneGraph() );

// Launch our app
$app = new t2dSceneObject()
{
   class      = "TestApp";
   scenegraph = scenewindow2d.getSceneGraph();
};

function TestApp::killOne( %this )
{
   if( %this.os.getCount() > 0 )
   {
      %obj = %this.os.getObject( 0 );
      
      %this.os.remove( %obj );
      
      %obj.safeDelete();
   
      %this.outline.setSize( %this.os.getWidth(), %this.os.getHeight() );
   }
}

$app.killOne();

function Mother::onLevelLoaded(%this, %scenegraph)
{
 
   $Mother = %this;
   echo(%this.getLinkPoint(2));
  }

function Enemy12::onLevelLoaded(%this, %scenegraph)
{
     $Enemy12 = %this;
   moveMap.bindCmd(keyboard, "w", "Enemy12Up();", "Enemy12Stop();");
      
}


function Enemy12Up()
{
   
   $Enemy12.dismount();
   $Enemy12.setLinearVelocityY(20);
   
}

function Enemy12Stop()
{
 
   $Enemy12.moveto($Mother.getLinkPoint(2),10,true,true,true,0.1);
}

function Enemy12::onPositionTarget()
{
   %i=$Enemy12.mount($Mother, $Mother.getLocalPoint($Mother.getLinkPoint(2)), 0, true );
     
   echo(%i);
   
   $Mother.setLinearVelocityX(20);

}