The above (or below, depending on votes) solution is excelent and gives you everything you need to know in order to get the achievement.
Problem is, you need to kill Ifrit to get the achievement, and I just couldn't do it, but searching the internet I found mention of this glitch and decided to make a video of it.
Here's the guide:
1. Make sure you have decent enough weapons equipped, and most important a decent crossbow;
2. Go to insomnia and start the fight;
3. Make sure to beat Ifrit untill he enters his second fase (goes up in flames);
4. Die and retry;
5. Watch the videos and place yourself in the same place, Ifrit will just stand there, it's possible he snaps out of it, but it's easy to get him back to being stuck;
6. I don't know if it's a glitch, but when you retry, Ifrit doesn't enter his third fase, at least not with me, and I've done this three times to make sure it works.
Thats it. Any suggestions I'm all ears.