THE EASY WAY
Although the other guides are fantastic they still rely on you having a significant amount of skill to pull them off. If like me you were only born with the thumbs of a mortal and not a Final Fantasy god, then there is an easier way.
Keep in mind this achievement not only requires you to beat Noctis, but you must also obtain an A+ rank. This will require you to defeat him as fast as possible (which shouldn't be an issue with this exploit), and obtain a combo of about 30.
Immediately upon the fight starting run backwards to the rear right corner, in between the caravan and the garden bed/parking spaces (there is a big Cup Noodle billboard behind this area for reference). STAY IN THIS CORNER AREA. Continue the fight as usual, attempting to get a 30 combo if possible. (I only reached a combo of about 26 and still got the achievement with over 1mil points). This portion of the fight will still require you to be adept at side stepping and Noctis will evade nearly all frontal attacks. I used the flame daggers and continually side stepped until he went into stasis then I attacked. Continuous side steps and short quick attacks are how to hit him.
Once Noctis loses approximately 30 percent of his health he will teleport to the upper canopy above the service station. Immediately run back against the rear wall and push Ignis into the corner against the rear of the caravan and the fence.
Although it may take some luck as it does not work every time, Noctis may stay on the canopy, walking towards the edge in a hobbling animation (as if injured). Since he wont jump down and attack you, you can hold (Y) and charge your true clarity meter. SELECT THE STORM DAGGERS. Once charged, step out and move slightly forward and to the right until your target icon locks onto Noctis. Quickly use true clarity which will hit for around 900-1000 damage. IMMEDIATELY run back to the spot tucked in behind the caravan. Charge your true clarity and repeat.
It is important to run back behind the caravan each time as Noctis will occasionally get hit for critical damage and get knocked over. This resets his animation and he will walk towards the edge of the canopy again. However, if he walks back in a slightly different spot you may come into his range and he will teleport and attack you. If this happens just let him kill you and restart the fight as he will not get stuck on the canopy a second time. I found that going back behind the caravan and ducking in and out of range will make Noctis less likely to find an opening to attack you.
You will only have to do this for 3-4 minutes, with the longest part of the fight being actually doing enough damage to get him on the canopy. Don't be worried about using as many health items as needed as once he gets stuck on the roof you will not need any more after that.
If this method doesn't work first time try a few more times as he will not get stuck every time, however, it does happen fairly frequently.
Well done to all those who got this achievement legitimately, but for the rest of us without a hope in getting this then I hope this works for you instead. Please add a like if this helped you out.