Cheng Yu knew his own killing prowess was limited. However, he realized his cooperation with Wang Yun was almost flawless.
The illusions he created could only be seen by himself. Take, for example, if he worked with others. It would be very easy for them to get taken in by his illusions just like the enemies.
If there were really a pit in the ground that he had covered up with his illusions, would a normal person instantly remember what the original terrain looked like during battle? So they would similarly get confused along with the enemy and perhaps might even lose their balance and fall into the pit before the enemy.
But Wang Yun was different. His memory was exceptional. When he first joined the Kong Consortium's intelligence agency, his ability shocked everyone.