Anyway I don't know if I can finish this objective. It have been there for weeks. Every time I made 3 or 4 of them, it is reset to zero.
Every time the door opens and the radio broadcasts, it increases Deathclaw chances. [This is a game tip on the loading screen, not conjecture.]
1) Build and man a radio room.*
2) Reserve one exploration slot (24/25) or possibly two
3) Arbitrarily pick a dweller and constantly send them back and forth through the Vault door.
4) When your other 24 Wasteland explorers go in and out of the Vault, never let them "share" the door. Make sure it is completely closed first, to maximize door openings.
Don't get me wrong, I feel ya, I've had that Objective reset at 6 out of 8 Deathclaw attacks, which took me days to get.
* We dont' know how the radio works to increase Deathclaw chances. Does CHA matter? Does the room level matter? Does having multiple radio rooms matter?
But if you want to experiment, you can build a 1/2/1 3x radio "room" and put 1 dweller in each of the 3 rooms.
If you don't want new dwellers, when it gets close to the 0:00 countdown, you can move them to other jobs and log off or leave them there for a long time if you keep playing.
If you have 999,999 caps, you can just destroy the rooms and build new ones.