You will be able to fight a Beach Guardian (Nadaig Glendemen) while doing Vasco’s companion quest named “Forever a Naut”. You’ll be able to start this quest a short while after finishing “Family Reunion” and talking to Vasco again. (You will also find a Beach Guardian during the main quest "Mystery of the Ancient Ruins" at the Ruins of Didri.)
This quest involves him asking you if you can take him to talk to Admiral Cabral in New Sérène about an honor mission for the Nauts. The Admiral will tell you about a Naut ship that was beaches and destroyed which has brought a negative reputation to the Nauts. She wants Vasco and you to investigate and prove it has nothing to do with the sailing capabilities of the Nauts. You will need to head to Wenshaganaw, “The Singing Waters” and then head West down to the coast. You will have to go through a Native village to reach the path down to a cave which will lead to the beach where the wreckage of the Naut ship “Oriflamme” is located. You’ll see some loot beams shining up above a couple of things you need to interact with to find out what happened. Things start to seem all too familiar to the giant beast you fought in Sérène. Once you get done investigating you’ll turn around to see the Guardian hiding inside a cave.
SAVE YOUR GAME before initiating the fight.
The Beach Guardian has a ton of health and a good chunk of armor. You’ll want to focus on the armor first so your companions can help DPS the Guardian. The Beach Guardian has four tentacles protruding from its face than fire electric bolts at you. You’ll see yellow lights light up in a square pattern from the tentacles when it gets ready to do this so make sure to pay attention and dodge quickly or they will easily eat your health up. The Guardian can also roll into a ball similar to an armadillo and charge roll towards you quickly and knock you down. If you see it curl up, get ready to dodge. It will also just outright charge at you for a headbutt, but that’s easily dodgeable. If you find yourself too close for too long the Guardian will start charging electricity around itself to do an area of effect burst. If you see any type of electricity forming up around the Guardian start backing up immediately until it fires off. Another move the Guardian will use is burrowing underground and then charging towards you this way. You’ll see it leave a trail of debris while it’s moving underground. This move is hard to avoid and can stun lock you if you’re not careful. I recommend unlocking from the Guardian as a target and sprinting away until it surfaces again.
Despite the Guardian having many high damaging abilities, if you stay aware and dodge methodically you should be able to defeat it with minimal damage or potion usage. Try and get the armor down first, preferably with firearms since it allows you to keep your distance. If you’re a mage I suggest using Stasis to freeze the Guardian and then using your Fury attack. Once the Guardian is defeated the achievement will pop.
If you find yourself having issues defeating the Guardian you can try changing the difficult to Easy in the game settings.
After you kill the Guardian you can head inside the cave it was hiding in and pick up the Captain’s journal. Take it back to Admiral Cabral in New Sérène then talk to the Naut tattooist to finish the quest and be rewarded with a Memory Crystal, 1,600 XP and +5 reputation with Vasco.