Fighting against an unknown enemy is a difficult thing to do and fighting against the one who knows everything about you is even more difficult. The Omega Force used to have a crewmate whom they trusted and he was with them when they were training. Later he just left them and selected his own path which was different compared to the one which Omega Force was asked to opt for.
Lucky was a sentient machine and he used to be a helpful tool when he was with the force. Then he became a ruthless killer who showed no mercy towards his prey. No one knows what had happened to him and what the circumstances made him alter his program in the opposite way. Now he is an enemy and the Omega Force has to bring him down because he is bringing death to the world.
Joshua Dalzelle i.e the author of the story has given a difficult assignment to Jason Burke and his men because they are fighting against someone who knows about them completely. It is like fighting your own shadow and Jason has finally received the most difficult assignment at last. Missions in Iron & Blood and Omega Rising were not that difficult because in those stories the main characters were fighting against an enemy.
Here the crew is fighting against a dear one who used to be their friend but he calls himself Seven now. Even if they succeed in reaching him still many of them would not be able to kill him. Perhaps this is the main reason that Lucky is alive and he has been able to kill that many people.
