For the vast majority of missions, I had the following formation:
Vanille, Lightning, and Fang (Vanille as leader)
Rav/Rav/Com
Rav/Com/Com
Sab/Rav/Sab
Sab/Med/Sen
Rav/Com/Sab
Med/Med/Sen
This formation helped me in the vast majority of missions. Now, something that can help you a lot are the potions. If you have any that you have not used in the course of the story, I recommend using them with the bosses that cost you the most, or if you cannot buy them in the store, but… they have an extremely high value (Which in desperate measures I had to use many times)
The 2nd formation I had for other missions was as follows:
Vanille - Hope - Sazh (Again Vanille as Leader)
Rav/Rav/Com
Sab/Med/Syn
Rav/Syn/Syn
Rav/Rav/Rav
Rav/Med/Com
Med/Med/Com
The previous formation served me mainly in missions when the enemy did not have such devastating attacks.
Finally, for mission N°64, I recommend having the crystarium at least up to LVL 3 of some abilities that your character does not handle, as if not it can get complicated due to the high speed of this character. Also, I use Vanille as Leader because her healing is very useful, so much so that it saved me many times from the screen saying “Game Over”
Vanille/Lightning/Snow (Vanille as leader)
Sab/Sab/Sab
Com/Com/Com
Med/Med/Med
Syn/Syn/Syn
Sen/Sen/Sen
Repeat Sab/Sab/Sab
Once I completed mission 64 and with all the other missions with 5 stars, I finally got the achievement. Most of the time, there will be save points close to where the bosses appear, USE THEM , and in case you have not completed the 5 stars, reload that checkpoint and try a different strategy. Once you complete mission 64, you will get “Gold Watch” which will increase the base time needed to get 5 stars, which is a relief to the heart in case you have to repeat any mission.
Note: (I don’t know if it is necessary to complete mission 64 again but in my case I had to do it twice, once without GOLD Watch and another WITH Gold Watch , when I completed it for the second time the achievement jumped)
I would greatly appreciate it if you could give a thumbs up. Greetings! and keep expanding the Crystarium (What a headache it is)