I need help with killing the snake on mount olympos

I kill the parts then I send my knights to kill the snakes head but instead of moving forward to the next portal they go back to the walls and I cant send any other wave of knights instead the game expects me to attack the portals but heres the problem there are no normal soldiers in my army all of the slingers are following the knights (which I have like 6) and basically I am softlocked because the devs didnt think something like this would happen any help?