Under such circumstances, they were all aware that in order to defeat Tang Wulin they would first need to design a trap for him which he would never see coming.
As a result, the trap was already being put in place ever since the first collision when the six Monsters lost.
Besides targeting him with their attacks, everything that they did was meant to confuse Tang Wulin.
The most important part of this trap was that every action that they had taken earlier was genuine. If they were capable of defeating Tang Wulin during the initial confrontation, obviously it would have been the best result, saving them the trouble later on. In reality, their tactics were also meant to minimize one of them, the least noticeable person among the Shrek Seven Monsters.
Other than his provision of Bloodthirst Bean Buns for everybody in the beginning, he had kept his presence inconspicuous.
He had been quietly waiting for the opportunity all along!