Story related. Cannot be missed.
Will unlock in Chapter 4: The Hecatonchires.
*** Spoiler - click to reveal ***Eventually you will reach the head of the Titan Hecatonchires who is bound with a platform in his mouth. Shortly thereafter Mageara will appear and infect the head which then sprouts prongs from his mouth, similar to the ones on Mageara’s back. The two prongs closest to the bottom of the platform are infected and vulnerable to attack. Fend off the smaller attackers—+ R1+Triangle is a good way to do damage to them all, push them back, while minimizing your damage intake during the many violent camera shifts—while destroying each prong as quickly as possible.
Once you successfully destroy one you will be launched to another bridge where you will fight another Hand of Aegaeon. However this time you will be able to mount it after it is weakened, similar to the Cyclops, and have it attack the head. Once you have done enough damage the Titan will bite down on the Hand and the trophy will unlock.
tips:
If you're doing this on hard and get stuck trying to get past the first part, I succeeded by ramming the enemies that bombard you (R1+X) while attacking only the right-side finger. After about 25-30 deaths, this is the method I used to get past this.