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10 G

For those who don't want to watch the video, what you have to do is get 10 hits while riding on the back of a titan.

As far as I know, the first/easiest place to get this is when you're trying to get out of the pumping area during the main campaign just over half the way though the main storyline. Joker sends one Titan and a few waves of henchmen for you to fight. Knock down the Titan and control it with the LS and RS sticks to hit the henchmen.