Getting to this side mission is not hard, it's located shortly after you move the posessed camera out of the way to open a security door, in the collapsed department area (I'll put a video at the end of my solution with the location).
When the mission starts (You will notice because of the scenario and soundtrack), you will face several enemies. Shooting with the standart service weapon or use throw ability for the flying ones, throw for the enemies shooting at you and shield for the heavy enemy until one barrier ahead of you take him down. Meanwhile you will have to jump and hover between two platforms (I don't want to give too many details and spoilers, as this mission is probably the best and the worst of the DLC, and this is because of the final boss)
Ok, so the last boss of this mission is the hardest one in the entire DLC, near Tomassini boss in the base game level of bad. He deals a TON of damage in each attack and doesn't stay still at all. Here is one easy way to take it down:
First of All, the Shield ability has to be upgrade at full, all of the skill tree. (Recommended to have Jesse fully upgrade)
Second, Pierce mod on your service weapon with as many DMG mods in it as possible.
For Jesse mods, I had max health and energy costs. You can have this one pretty much at your personal choice, but there is one mod which gives you healt in exchange for throw hits that can help you to arrive at the boss fight with more health.
The key word in this showdown is DON'T BE GREEDY and don't try to land too many shots, as one single hit can pretty much put you at the starting point again. If he kills you, you will have to undo the path from the last control point and do all the side mission AGAIN. When you see the health barrier of the boss, locate him and use your shield. WAIT until he makes his two attacks (You will notice he has no more stones to throw at you). Then, use your pierce mod of the service weapon and shoot him twice. Rinse and repeat. Again, don't be greedy and have patience. With this method he will not last long (I killed him in less than two minutes this way). I don't recommend to use throw with him (He will dodge nearly 95% of this attacks and it wastes energy for the shield)
To finish this, here is a video of how to get to the side mission. I don't take credit for the video: