- Stay out of combat
- Use guards as a human shield
Fast-travel to Havana and head to the mission marker to start this mission. All of Havana will be under curfew so the guards will be hostile on sight. Head to the marker and find Torres. You won't find him but his right-hand man, El Tiburon who will be in the rear entrance of the church, near the fountain.
You'll now have to tail him as he and his guards are making rounds in the town. It will be easier to track him on the rooftops. Just be careful of the gunners. El Tiburo will return to the fort northeast of the town. You now have to find and kill Torres. As an optional objective, you have to do it stealthily.
If you've entered the fort before, then you should know where you can sneak from. If not, check your map. You'll have to enter the fort in the same corner where the viewpoint is located. Make the nearby guards to berserk or put them to sleep as you slip pass through them and reach the fort walls. The more guards you subdue, the easier your job will be. Take your time quietly taking out the guards; it will be better than reloading your checkpoint. Prioritize taking out the gunners first since they have longer sight range.
After all the nearby guards are taken care off, cross the rope and wait for Torres to walk underneath. Finish him off with an air assassination or rope dart.
After discovering that you killed an imposter, you'll have to kill El Tiburo next. Your secondary optional objective will become available as well. Counter their attacks and throw but don't kill any guards yet. Wait for the prompt to use any of them as a human shield and do this two times to complete the optional objective. El Tiburo will be immune to almost all your other weapons and the best way to deal with him is by shooting him. You can run around and hide to reload then shoot him again.
Escape the area and hide to lose your heat. Once you lost your pursuers, report back to the bureau to complete the mission.