My Night Job

This trophy is cumulative and can be done over multiple plays.

In order to get the rusty key, you must first find the grave keeper (See Six six six for map) and then get him to a helicopter spot without him dying. When he is safe and boards the helicopter, he'll give you the rusty key. The rusty key unlocks the door to the room seen below:

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Go to the room and unlock the door. Rinse and repeat 20 times and the trophy will pop after the final one.

You can see how you are progressing by going to the trophy room and going to the spot where that trophy will be (9th from the right).