RE:R2 has many, somewhat grindy achievements that require you to kill enemies in a certain way. One of the more difficult ones, if not the most, is Unfriendly Skies. Your objective is to shoot down 30 enemies mid-jump. A "normal" way to get this is by fighting enemies who carry melee weapons (not unarmed) as they sometimes jump while trying to hit you. Due to the erratic, unpredictable movement and speed of the enemies, added with the fact that they rarely jump, can make the achievement very difficult to get. However, there is a simple way to farm this achievement.

• The DLC Episode Two: Contemplation. Buying the game without any DLC will only give you the first episode, so I recommend buying all episodes for the full story.
  
• A powerful weapon that can kill in one hit, like a sniper rifle or magnum.
  
• Ammo for that powerful weapon. At least you must have one bullet.

In Episode 2, after escaping from the village, Neil will accompany Claire and Moira for a short while. After some time, you will have to climb a broken ladder to a balcony. First Claire will help Moira up, and Neil helps Claire. However, zombies broke in and chases Neil. He will run away, and zombies will follow him. Turn around, there's a door. Enter the door, and you will be in this hallway:

Follow the hallway and enter the next rooms. Do not collect anything since we will do a lot of reloading. The next room is a storage room, go to the next door and you will be at a restaurant. Notice the item box on the left, the one that only Moira could open.

Move to the item box. You will see a zombie standing on top of a beam across the room. For some reason, shooting this guy counts for progress despite him just standing around there. Pull out your powerful gun, and shoot the guy in the head. If you use a weak weapon (like a pistol) he'll jump down, and it won't count for the achievement.

You can track your progress by pausing the game, opening Gameplay Stats, and choose Records. Scroll down, and you will see the Unfriendly Skies record. Keep on reloading the checkpoint until the achievement pops.

If reloading that part again and again for 30 times may seem boring, it is the easiest way. However, there are 2 other parts in the same episode where you can use to progress the achievement, even though you might not get much. Both of them are in the village.

The first is where you need Pedro to drill a wall to get the fuel. While he is drilling, turn around and look at the house behind you. Enemies will spawn at the roof, and they will jump down to you. You can shoot at them while they're jumping from the roof.

The second location is on the roof of the bar, after Neil lowers the ladder and tells you to climb. Instead of following him to the right, go to the left where the part of the roof is lower. Stay there, and occasionally enemies will jump when moving from the higher roof to the lower roof.

Credits to GreenDan in the comment section of this YouTube video for finding this really easy method. It also shows a good location to farm the Disarmed achievement.

If you know a location and/or method to better farm Unfriendly Skies or Disarmed, I would love to hear it.