General objectives: It may seem that eliminating Dijana at the beginning of the mission is a good idea, because you encounter her almost right away. The best option however is to kill both nuns at the same time by using objects from the environment to your favor.
Dijana Radoncic is patrolling the entire area. You will encounter her standing close to the reception when you start the stage (screen above). Naturally you can use this opportunity to get rid of her right away. One of the possibilities in this case is using instinct mode, however you must make sure that a guard standing next to her dies as well so he won't be able to raise an alarm.
An even better way of neutralizing Dijana is by "outsmarting" the game's mechanics. Take cover behind the reception's counter, get closer to the nun and perform a silent takedown by dragging her over the counter (screen above). This action (rather unexpectedly) shouldn't be noticed by the guard Dijana was talking to.
If you didn't make it in time to take care of Dijana or if you've chosen to remain unnoticed, then she will begin travelling between the reception and a gas station seen in the distance. You should move towards the gas station either way, because Agnija Radoncic can be found there. She's the second target and if you'll make some preparations you'll have a chance to kill both nuns with a single action. The safest way to get closer to the gas station is to choose a path to the left of it (screen above). It's the same place where you could intervene and prevent one of the Agency soldiers from assassinating the captured motel owner.
Be careful here, because the area separating you from the gas station (depending on the difficulty level) will be guarded by one or two enemy units. If you have to deal with two soldiers, then start by attacking the one that's on the move (don't forget to use a nearby radio and throwable objects to your advantage).
Eventually you should be able to reach a point displayed on the screen above. You can see Agnija Radoncic from here. If she's the only target left, then you may use a Silverballer to shoot her, however you must make sure that no one is looking your way so your cover won't be blown.
Another idea for getting rid of only Agnija is to turn on the radio found in the recently secured area of the map. This should convince the nun to inspect the problem (screen above), allowing you to surprise her in a safe place.
The best idea by far is to get rid of both nuns at the same time. Start off by making sure that you're stationed to the left of the gas station. Wait for a good moment to leave cover and proceed towards a single gas pump (as displayed on the screen above). Interact with the fuel distributor so that gasoline will start pouring out of it.
Return to the recently secured area and wait for Dijana to arrive so that both nuns will be standing close to the gas station. Use the silenced pistol and shoot at the pool of gasoline, causing an enormous explosion (screen above). Important - Don't worry if one of the nuns doesn't die instantly, because your action will trigger additional explosions of cars parked nearby and sooner or later she'll go down as well.