General objectives: LaSandra should be the last person to die and she's guarding the main farmhouse where the mission ends. Killing her is not going to be easy, because the farm buildings are being guarded by plenty of Agency soldiers.
The last person you need to kill is occupying the main farmhouse seen on the screen above. It's best to approach your current destination from the left side, using a smaller cornfield to avoid being seen by enemy units.
There are two recommended ways of eliminating LaSandra Dixon. If you don't want to raise an alarm, then you should start by taking cover in the area seen on the screen above (between two farm buildings).
Wait for the nun to make a stop close to the cornfield and shoot her with a silenced weapon. Immediately approach LaSandra's body and start dragging her. You can hide the body in the cornfield or in a nearby toilet (just make sure that it won't be found by any of the guards).
The second recommended method of neutralizing LaSandra Dixon is to stage an accident involving the nun. In this case you must begin by heading towards a large barn seen on the screen above. There's a single guard nearby, however you can easily sneak past him.
If you're playing on lower difficulty settings, then you can enter the barn right away and immediately interact with a ladder (screen above). On higher difficulty levels this isn't possible, because the ladder is being guarded by an additional person. You can try to kill the guard or distract him with a throwable object (in this case the best course of action is to convince the guard to leave the barn and make his way to the cornfield).
Once you've used the ladder you'll find yourself standing in front of a crane. Wait for LaSandra to stop under a heavy load and shoot at the chain in order to go through with the unfortunate accident scenario.