Assassinate Taj Taheer
Taj Taheer is at the airport and you’ll have a hard time permanently tagging enemies since there’s a jammer. In some ways, the jamming vehicle actually works in your favor since you can repeatedly tag Taheer whenever you need to trigger an autosave.
Taheer initially gets out of the jeep, and he’ll bunch up close to the two heavy bodyguards. If you have explosive rounds, you can take out all three to complete a challenge. However, this will put the entire base on high alert.
Note: As far as I know, this is the only time Taheer moves near the two heavies (outside of trying to use luring bullets on them).
Conversely, you can try other challenges such as killing all the soldiers at the airport (and one sniper at the mountains to your left) without raising the alarm. Use the generators and fuse boxes to distract some of them if need be. Again, tag Taheer with your binoculars if you need a quick autosave in case you make mistakes.
Note: After leaving his jeep, Taheer goes inside the tent, then he walks to the people loading the plane. He’ll go back and forth these areas. Moreover, if you’ve reloaded a few times, the corpses will disappear and Taheer won’t notice them.
As for the challenge where you need to kill him while he’s inside the plane, I believe Taheer goes inside it after you’ve eliminated most of the soldiers (including the bodyguards and snipers). The plane just won’t take off since the soldiers loading the cargo are dead.
There’s also a visual bug here. You’re told that you’ve got a tight shot to snipe Taheer through the plane’s door (which has a glass window). Unfortunately, the texture is completely opaque. However, if you have armor-piercing (AP) rounds, you should be able to see the red dot signifying that you’ll be able to hit him. With a bit of luck, you might strike him in the head too.
I should mention that you might want to put a few traps, decoys, or turrets behind you just before you assassinate Taheer. Once he’s dead, several soldiers will spawn near your position (don’t worry since they won’t ruin any “kill without alerting anyone” challenges).
After taking out these hostiles, simply head to the exfiltration point (you might spot a couple of baddies along the way). That’s it, you’ve completed the campaign of Sniper Ghost Warrior Contracts 2.