Saar was the City Lord of a Basic City, and Meng Tian was the City Lord of a system main city. With their combined strength, they were able to stall Shi Jian – this was why Zhao Fu didn't care about Shi Jian chasing after him, as he had made sufficient plans.
Wang Jian, Wei Liao, and Bai Qi were stationed near the three other system main cities in order to prevent the three other City Lords from helping out.
If there hadn't been a need to prevent the three other City Lords from interfering, Zhao Fu could've gathered all of his City Lords, lured Shi Jian out, and killed him.
This was because Great Qin had six City Lords including Zhao Fu – with six of them against Shi Jian, Shi Jian would definitely lose.
However, that wasn't Zhao Fu's plan because he also wanted to obtain the Fruit of Life Tree.