Silver Knight managed to reduce some of the bolt's power. But the demonic energy put on it by the Flesh Golem was too much, and Silver Knight couldn't do enough to save everyone around them. The best she could do was reduce the casualties by plunging the bolt into the ground and halting its advance as much as she could. However, that still left several of the people in the front dead since they took on the brunt of the attack when the bolt pushed through the ground and into their formation.
The force and shockwave of the bolt were enough to push Silver Knight and their division backward. And the ones that survived fell to the ground or held onto the ballista for balance. Most of Silver Knight's special attack division survived. But the bolt had ruptured the ground where the ballista stood. So, Silver Knight had to move it to a better place before they could begin shooting again.