One could think that, when surrounded by a group of people using guns, it is impossible to pull through. In fact, when you are a normal human, without high speed, good reflexes, or amazing strength, that's generally the case.
Casper was aware of this fact from the start. However, now he had the power to overcome this.
The soldier had enough speed to close the distance between him and the Frelsi and begin his rush among their lines. It was easy to deal with various armed enemies that were in a closed place like the throne room when one could short the distance before being shot. This way, they wouldn't be able to open fire on him, because they would surely target an ally once they did so.