It's the HUGE One, This trophy is story related can't be missed
This should be your'e forth trophy. This is your'e first boss battle against the two little hydra's and the left and the right go for one of them keep attacking them and block when you see there head going down as that means there going to attack you often the two hydra's will suck you left and right attack them both and one will go down the left or the right at the far side of the boat they will be a set of box's you can jump up quickly get the top and next to you will see a circle platform jump on that and it will go down and hit the hydra and keep him down. but be fast do the exact same for the one on the right. Now that's two down one more to go climb up to the top and fight the biggest hydra it has 2 attacks. Be sure to keep rolling around so you don't get bit when his teeth are in the wood attack. His second attack is the scream all you have to do is block and you wont fall down on the lower deck keep doing this and his head will slam down as he's hurt and this is just a button mash part which it's only three part's as you see it breaks the mast after three times it's broke keep attacking him the fourth time his head wont slam down he just shakes his head side to side go up to him press O a small qte will come up after that it turn into a button mash after that a cut scene will play as the hydra's die. Go into the the big hydra's mouth at the very end there's a guy trying to get back up grab the key and the trophy will pop