S.T.A.L.K.E.R.: Shadow of Chornobyl

As a preface, DO NOT JOIN FREEDOM BEFORE STARTING THIS, if you do all the ecologists will be hostile towards you and you cannot complete this questline. I would hold off on joining them until after you complete Lab X-16 as you don't need the ecologists for anything after that.

As you go through the Wild Territory, you will notice bickering between the Mercenary leader and Professor Kruglov. When you get to the trainyard a cutscene will play with Kruglov asking you to help him. Kill the mercenaries on your way to Kruglov and he will then ask you to escort him to the mobile lab in Yantar. You will be attacked by more mercs on your way, just make sure they do not kill him. I would save often just in case he gets killed so you don't lose much time. The escort quest will end after the underpass full of burner anomalies and he will make his way to Yantar. You should also head to the mobile lab in Yantar to get the next story quest involving Kruglov, as he must also be kept alive for this quest.

This next quest also involves you escorting Kruglov, but now the enemies are zombies and sometimes some snorks. Kruglov will stop at two points to take readings, and right after or during the second reading make a save. There will be a blowout soon where you and Kruglov will be unconscious for a little bit, and there is a chance a zombie will kill Kruglov while you are still waking up. If this happens just reload the save and try to make sure there are absolutely no zombies (and sometimes snorks) that can kill him in the area with the broken bus. After you wake up, talk to Kruglov to get him up and he will take the same path back to the lab. When he is inside he cannot be killed and the achievement should pop.

Some other bugs to be aware of according to the wiki:
- Sometimes Kruglov will not follow the player and behave like a regular NPC, this might be caused by saving before he asks you to save him, so don't save before you fight the mercenaries, otherwise you might need to reload the autosave for entering the Wild Territory. It should be safe to save after you have started escorting him.
- Sometimes a pack of snorks spawns at the burner tunnel entrance, and can overwhelm Kruglov.
- When Kruglov gets to the other side of the burner tunnel he will stand still sometimes, this can be prevented by killing the zombies in the tunnel before he passes through it.

This questline can be a little tricky, but with some persistence and some beforehand knowledge of the bugs you can get through it. Good hunting, stalker.