An easy way to do this one, while not the most legit is to Death spam on the Neochu, (mission 55 I think) and on the Adamantoise (mission 63). Those two were the hardest I think. For the multiple Zirtirnia (sp?) you have to fight, use Snow as a sentinel and keep him that way for the majority of the fight. Have your other two characters (I used Lightning and Fang) fight as Ravagers until his chain gauge is up to 400% or so, then have either Lightning or Fang use Army of One or Highwind, respectfully. For the multiple Tonberry fight (don't know mission #) use a Deceptisol before the fight with a Cerberus setup (3x Commando) and spam Ruinga it took me like 17 seconds to finish the fight.
For Vercingetorix, (mission 64) I had each of my characters equipped with at least 1 kaiser knuckle (strength boosting accessory), While my main had 4 equipped (Lightning for the record). Use Commandos in the beginning, because he is going to go into his barrier too fast to build any chain gauge. When he starts using Wicked Whirl, use the Tortoise paradigm (3x sentinel), then heal quickly. at that point he should stay out of his barrier long enough to build a chain gauge. Once you get it up to 250 - 300% go back to your Cerberus paradigm (3x Commando) and beat the tar out of him. It took me a little over a minute and a half this way.
Some people will try to use the genji glove, or build the chain gauge higher during the fight. I have found that this takes longer in the end. Just my advice on a few of the harder fights in the game.