As you can see, Alex already cleared the fifth portal but the thing was that after that he already noticed Monster already began to come out from those other portals so naturally at that time he won't be able to use the starfall technique in those places. But he already planned something and at the same time he created more clones.
Naturally the main body already created the tornadoes in front of the portals from where you can see those monsters coming out. He sent that one clone inside of that portal somehow while using the ground Slam technique. Well, as you know the Boss monster will naturally take time to come out of the portal so at that time he wanted to send the clone inside of the portal and after going inside of the total that clone naturally uses the star for technique and instantly deactivates himself.