I have come to the conclusion that the people in the world being the most difficult to kill overall are those belonging to a racial subgroup concentrated in an area with a tactical advantage (e.g island) as well as having a high level of general physical attractiveness, to deter people from supporting relaxed requirements for assimilation, which would facilitate enemy activity. People who are part of a very culturally sophisticated (e.g refinement, promotion of family unit, etc) and generally highly critically-minded tribe, which is racially and culturally homogeneous and has heavily fortified and internationally widespread residential territory as well as rigorous and infallible systems of preservation in place. They must have access to and training with modern weaponry, preferably magazine-fed rifles (preferably ACRs with attachments) because they are powerful and versatile, pistols and large combat knives for close encounters, as well as durable body armour. They must also have their own media, with all external media unavailable through television, with strict safeguards against the spread of cultural degeneracy, with a clear and unalterable list that determines what qualifies as this.
They must also have access to apocalypse shelters, with measures for sustainability, to survive a catastrophic event and rebuild civilisation. They must also have control over the state, with appropriate legislation as well as constitutional amendments brought into effect to make it as difficult for hostile elements to repeal said political features. Regulatory bodies must be formed to stay vigilant against all forms of subversion.