Okay, so i got back in the swing of things today by playing for the first time in 3 months. I'm really happy cuz I whooped the Gigantaur (though it was a 0 star rating =\) and i beat one of the zirnitras. So ive got one zirnitra, the fucking adamantoise in sulyya springs, and fucking VERCINGETORIX!!!! THAT BASTARD IS IMPOSSIBLE, I SWEAR. Ive got the guide and everything and im just like "FUCK THIS"

Here's what I did: 
 
Use Vanille as your lead and fight similar to how the guide tells you.  Just keep healing, buffing and debuffing.  Make sure Poison is constantly being applied. 
 
Now for the cheese....  When he's good and debuffed and Poison is applied (The important part), cast your summon and quickly go into Gestalt mode before he uses his invincible aura.  You want to drag Gestalt mode out as long as possible so the Poison keeps draining him.  Keep using Chain Cannons right before the meter hits bottom.  Chain Cannons only cost 1 point in Gestalt mode so you can take up more time. This is why you use Vanille.   Hecatoncheir is the only Eidolon with an 1 point Gestalt ability.  By the time your summon runs out Poison should have taken down about 1/3 of his life.  If he uses invincible aura during your summon you'll probably have to restart, which is why you want to get him good and debuffed before you summon and quickly go into Gestalt once you do.
 
To 5 star the fight, I took him down to about 9,000,000 HP using regular tactics, used the summon/poison trick, followed up with an Elixir to refresh the TP meter, and used another round of summon/poison.  Incredibly cheap and easy. 
 
Here's a good youtube video that shows paradigm setups and whatnot also:  http://www.youtube.com/watch?v=Xb9RDzO86vc 
 
Hope that helps.

Use Vanille and Death. Simple as that. It works. 
 
For Vercingetorix though, I used Snow, Hope and Vanille. Snow was the main tank and was always provoking him and sheilding, while Hope was the main Healer and Vanille did the above Poison thing with a bit of healing when you get the chance. I five starred the fight without using any summons or items, but your timing and knowing what he's gonna do has to be impeccable. Just keep fighting him and know when you can cast your poisons, your heals and your shields. Also, Make sure you have these paradigms ( order is Snow, Hope, Vanille):
 
Consolidation: SEN MED SEN (When Vanilles Health is low only, make the same paradigm with Hope as the SEN and Vanille as the MED for when Hope's health is low)
Salvation: SEN MED MED (For when Snow's health is low and/or you have some time for some major healing)  
Evened Odds: SEN MED SAB (To inflict the poison while Hope heals)  
Tortoise: SEN SEN SEN (For when he uses that attack that can pretty much kill you one hit, immediately after he finishes go into Evened Odds)
 
Another thing I forgot is that there's a spell that makes the enemy more susceptible to magic but I can't remember the name of it. I'm pretty sure there is anyway. Use that before you start poisoning and make sure your timing is good.