Before attempting to get the "Guessing Game" achievement, you should buy the following skills: Chameleon, Dominator, ECM Overdrive, and Shinobi. Other stealth skills can help, but these are the most important ones. A weapon with a silencer can also help. In the Firestarter (Day 2) mission, there is a random amount of wire boxes in the FBI building. Two of them will disable the alarm of the room you need to enter. You have to be very lucky and cut the two correct wires. There is absolutely no way to tell which ones are the right ones, and you are not allowed to use the computer. The boxes change each time you replay the mission. If you cut the wrong wire, the alarm will go off, and you should immediately restart the mission. The wire boxes spawn randomly. You should be able to get the right ones every 10-20 playthroughs. If you find the two correct wires and no alarm goes off, you can proceed. If any guards are in your way, use the Dominator skill to make them handcuff themselves. Go to the server room, and pick the lock of the door. Inside the server room is another door. Use an ECM Jammers (requires ECM Overdrive skill) to silently open the door. Grab the server, and bring it back to your vehicle without being seen. On your way out, you may want to use your second ECM Jammer to disable the cameras. Note: The mission can be played on any difficulty. Day 2 is the same for all difficulties.