Day 3: Chen's Plans
Primary Objectives: Defend the Mainframe / Defend the Safe / Defend the Datakey
Secondary Objectives: None
This one is a series of defense objectives, which it may not seem like it but are actually easier to do than the attack missions since you only have to succeed at one of them to win the mission! Obviously it is easier to defend as an Engineer since they can deploy landmines and Gatling Turrets -- and I highly recommend that you do!
Keep Security from Uploading the Datakey
Chances are you will not be able to keep them from hacking the mainframe because of the structure of the room and their ability to toss grenades into it. The bar though, that is a different story. If they do get the safe open all is not lost, in fact keeping them from uploading the data is the easier of the three tasks -- just run ahead to the midway point to the terminal, set up your Gatling Turret and some mines, and then wait for them to make it to you -- if they can.
Your team is pretty good about taking down the key carrier -- so you may not even need to battle at all!