Tiamat - Boss fight. First you need to throw an inert bomb and stick it to her. Then you will need to use your thrown weapon on the fires then on the bomb thats stuck to her. Use the dash ability to the left or right to dodge her fireballs. When she crashes to the ground unleash hell on here, generally around 10ish hits. Then when she flys up, lock onto her, and stay far away when she charges, repeat until she comes down to the ground to fight.
Once she is down on the ground there is a few things that she does. She crashes down to the ground, this is usually after you have beaten on her a little with a small combo. Good way of dealing with her is to use your blade geyser then get a few hits in and dash away twice orthree times. She will start with the fireballs again from the ground, simple dash left or right will deal with this. At a certain point she will charge you, and the B button should appear. Hit B a few times, if you do it right then War punchs Tiamat, do it wrong and you see yourself getting burnt from the air. Rinse and repeat, not only should you end up with the achievement but also a lifestone as well.