This is a tip for all missions,, not just mission 64 as I see most people focus on. also as just my opinion.. mission 64 isnt even that hard, beating it will almost garentee you 5 stars.
anyways, as for what I had to do with some missions being really hard to 5 star due to low target time is simple, kill adamantoise / Long gui for platnium ingots if you need the gil for Deciptol, which sell for 30,000.
Use that and the other shrouds for buff before any mission you struggle with and you should be able to 5star them all no problem.
If you do finish Mission 64 you will get Gold Watch which extends battle target time, this will be of great help aswell.