Continuing from previous story achievements, you will get to a point where you have to connect power to the four outposts. You can check the map in the powerline tree and plot the best routes to the four locations. Tips for unlocking and grinding will be in other guides as they relate to various achievements.
Once you unlock all four outposts, you will be entered into a new story related sequence. The achievement however won't come before a tough battle with.....
This tough foe is no pushover. I recommend being at least level 20 before attempting him, as I was at 23 and still had a rough time. There are two big tips for beating him:
First is stay behind him at all times. His feet and tail are his major weakspots, and you will have many chances to attack, just don't get caught in his sight as his blows are brutal. While his attacks are strong and range is long, he is decently slow. After attacking, he leaves himself open for multiple attacks. Don't get too greedy as he will follow up with strong nearby attacks if you dont warp away. His worst attack is his spinning move. Keep an eye out for this move as he starts to wind up his sword. If you and your AI teammates get caught in this attack, the run might be dead.
Speaking of AI teammates, KEEP THEM HEALED. This is the second biggest focus. Keep an eye on their health at all times. If they go down, dash and revive them and then dash to get out. As long as you keep them up, this fight will be long but straightforward. Avoid grouping up as his moves can instantly take down your whole team, and then the run is likely dead. Each player gets one fall revive, so avoid death at all costs.
Other than that, enjoy the boss fight and the nice cutscenes that go along with it.
EDIT: If you are still having difficulty, another strategy I discovered is playing defensive. This method is very long but relatively safe and I found it to have high success rates. Make sure you have the healing light sigil equiped. Instead of getting close and personal with the boss, focus strictly on healing your teammates. They will do the damage to the boss, while you ensure they don't fall. Again, follow most of the steps above by quickly dashing to save them, then dashing away. This method will take around 15-20 minutes around level 25, but if you can't beat him head on and want to get the fight over this should work.