The easiest way to get this achievement is to complete the game on Easy/Normal, clearing out the map, getting skills to level 5 or 6, etc. first. then start a new Legendary world - you can get this achievement in 30 minutes.
Step by Step guide:
1. Go back to the main menu, select Solo or Coop Campaign, select your agent and a new World set at Legendary difficulty.
2. Skip all the cutscenes by holding X. You can fly through the first 2 power stations and disconnect the batteries, don't even worry about enemies. Then go and fight Berg, I spammed him with the Anubis for a few seconds and he was down.
3. Now you're in southern Westport. Call your agency vehicle and take the highway east, past the monorail station and across the bridge to Southern Heights. When you get to the fork in the road, turn left. Now follow this road allll the way into New Patheon and onwards, it will curve around right and get you to the higher level. Unlock the Supply Point "The Citadel" just in case you die. Pick your weapons before you leave, I personally used the Anubis, Decimator, Oblivion, and Singularity Grenades.
4. Follow the Youtube video below on how to climb the tower. You don't have to drive the car along the pipes if you have high enough strength, just pick the car up and take to the end of the pipes, get in and jump on the wall.
5. When you get close to the top, open your map and highlight the highest Supply Point "Terranova HQ 3rd Floor (Security Sector)". You'll want to get out before reaching the very top (at around the 2:08 mark in the video) and jump over to it, it's easy with level 5 agility. As soon as you hit the Supply Point it will teleport you to the boss fight.
6. On the left side of the rooftop is a sloped part you can walk down and hide. Most enemies can't see you, if they start getting too close just throw a grenade or use the Oblivion to take out the groups easily. Use the Anubis or Decimator on the dragon when it flies over. You can make a quick dash to ammo when there's not too many enemies.
7. When the head is exposed she drops gas canisters all over so clear some out if you need some room to move. She also sends out pulse wave attacks, either jump over them or take the hit, they don't do too much damage. Attack her head when she flies towards the roof and the achievement will be yours.