This is not a solution to this achievement, but a warning.
If you are going in this order of generators:
1,2 Right side up and down
3,4 Middle (garage)
5,6 Left side (Gas/Petrol station)
When you are in the garage doing the generators there, DO NOT KILL THE ANCESTOR AFTER THE GENERATOR HAS COMPLETED. If you do kill the ancestor after the generator has completed its powering up you will end up glitching the game and you will not be able to trigger the ending with Cross. The dialog will stop and the second ancestor will not spawn at the Gas/Petrol station and you will have wasted your time.
We have done this in three different sessions all experiencing the same problem. Just be careful. If the ancestor dies during the powering up that is fine, but if they are starting to leave you should not kill them.