You do not need secondary roles at all for that battle. You don't even need to upgrade your weapons for it; I did all of the hard late-postgame missions with tier-1 weapons and, believe me, it's better that way. You can get through it easily.
Here:
Vanille/Fang
SAB/SAB* (start with this)
SAB/COM
MED/SEN
RAV/COM
RAV/COM (ATB cancel)
MED/COM (just in case)
All you need to do is showcase how well you've become accustomed to Vanille's first three jobs: SAB, RAV, and MED. That's it. You just need to boost that gauge in the corner. You don't have to take its HP all the way down.
Equip accessories that boost HP, MAG and Earth defense on Vanille at the very least. Equip her Belladonna Wand for better luck with debuffing.
This battle is so easy, but people aren't really used to using Vanille at all, so...
1. Debuff the enemy with Vanille and Fang. I would recommend manual input of abilities. It's weak against Deprotect, Deshell, and Imperil. At least try to stick Deprotect and Imperil on it.
2. After you stick debuffs on it, attack it with RAV/COM and SAB/COM (especially if you're trying to still land debuffs on it). If your friend knows how to ATB Cancel, use it.
3. If it hurts you pretty bad (like, 50% or 60% of your health) switch to MED/SEN or MED/COM and heal up.
4. Rinse and Repeat.
Hecatoncheir's attack pattern is basically:
-stand still (best time to debuff him)
-attack (best time to maybe switch to MED/COM or MED/SEN)
-stand still (attack or debuff him)
-attack (see previous description)
-stand still
...you get the point. His gauge boost depends on what Vanille does in battle.
If you still can't get things done, then the best and easiest place to farm CP at that point, though, is in the cave you were just in. Go back to the beginning of it, and where the path forks.
Here's a map; right where it says Juggernaut is where you're supposed to go (but don't fight it yet because you'll get creamed). Just go back and forth along that path to revive the enemies and fight them over and over (the robots and the bombs).