The plan wasn't very different from what they did to the other Ruin City floor; they needed to draw the Player Guards away. But there were two problems with this plan.
First, these players had no chance to be beginners like the ones he encountered on floor 11, so they could be more knowledgeable and read his deception. So he couldn't use his Golem this time.
The second problem was that the guards weren't one or two but close to a dozen, and all were very near the stairs. Most were sitting on the steps, but some moved around the City, investigating if something had changed.
At all times, at least six guards were on the stairs, and different groups were investigating the City each time. Shingi had spent some hours the other day checking how they worked, so he understood them well.
He usually would spend days monitoring their actions and making a perfect plan, but he didn't have that much time.