The Tyrant was over three meters tall. The top of Big Pineapple's large alloy shield was, at most, beneath its chest area. It was quite impossible for Big Pineapple, as a Heavy Armor Warrior, to prevent it from moving forward since his skills were all in cooldown.
However, Big Pineapple wasn't really that worried. On the Death Hunter Island, similar situations appeared when they fought the Hunter T-750. However, in those situations, Feng Luo dealt with those situations without receiving any injuries.
Not to mention that Big Pineapple already knew that Feng Luo had awakened the Force. He even knew that Feng Luo was the crazy gunner player in the video posted on the forum that swung the Tyrant as a weapon. So, he did not worry about the backline at all.
However, it was only Big Pineapple who knew this, the other three agents did not.