There are many special achievements in "Stalker 2: Heart of Chernobyl", and killing them all is one of them. The requirement of the achievement is to kill one of each mutant, so many heads, one wall. No less! The difficulty of this achievement is still very high, after all, you don’t know where the various monsters will be.
How to get the Kill All achievement in STALKER 2: Heart of Chernobyl
Kill them all
Kill one of each mutant. There are so many heads that can’t fit on one wall!
Hunt at least one of each mutant type. You can wander around each area so that you can easily kill all the mutants. After all, the probability of mutants appearing is still very high.