I found it easy this way if it helps anyone
How to get A+ rating
1. First aim for 30 or more combo. Its a must!
2.when attaching don't tap
hold it down so you can easily counter
when promoted to do so.
3. In order to get the 30+ combo you want to attach straight away as shown in the video until Noctis is in stasis. He wont be able to attach until he recovers for a little bit. This is the best chance to aim for the 30+ combo.
4. When Noctis gets into stasis and you're attacking him, remember to flick your left stick any direction while still holding
. This will make you avoid his attacks and let you carry on your combo until you hit 30.
5.Once you have hit 30 combo take Noctis to where the truck is and hold B while countering whenever possible. Don't spam your potions, use them at a good pace. You should mostly rely on your counters to recover any health lost.
6. Dont use the high jump whenever you want. Save it for when Noctis uses his Armiger chain because otherwise its impossible to avoid it. Therefore, as soon as he uses Armiger chain use your high jump 3 times and he should be finished with the chain by the time you used it all up.
7. You can even use Total clarity if you dont have enough high jump to avoid Nocts Armiger chain.
until his dead and at the end you should get A+ rating and the achievement :)