Ok, I guess you are referring to the Sentinel when falling down out of the sky. Are you sure you killed both engines already? You have to catch up with the falling Sentinel a number of times, each time killing part of the Sentinel. Once you have killed both engines, you get a final showdown, where you indeed need to mash your 'O' button. What I do is hold the controller with my off-hand (right hand for me, as I am a leftie), and repeat-mash the 'O' with my other hand. This way, I can mash MUCH faster than when I simply hold the controller.
This SHOULD get you to finally kill off the Sentinel, it is indeed one of the more frustrating parts of the game, but all the more gratifying when you finally kill him off.

Hope this helps.

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