Go to Shuri Castle on campaign.

After you get supplies from the drop and the Japanese ambush you, proceed on until you notice more Japanese in spider holes.

To get this right you must be a good shooter, but it should be easy.

The Japanese will lift the trap door a little.

When they do, shoot them once, preferably in the head.

The door will drop and you now can drop down into the spider hole with the door on!