Start with Sen X3 to eat the Ultima.
After the Ultima switch to x3 Med if needed, might depending on what % magic def you have.
When healed switch Fang and LIght to Saboteur and Sazh to Synergist. Land Slow, De-pro De-shell. While Sezh is at the least casting Haste, Pro, Shell on everyone. I normally keep that set up untill Fang gets Bravery.
Now that everyone is buffed and the turtle is enfeebled I switch Sezh and Light to Rav and Fang to Com and let loose on it.
Only switching from this for Ultima / Quake / Scream, and going X3 sen before it goes off the to med when needed. I like to keep everyone in green.
When staggered go Fang, Light on Com and Sazh still on Rav and kill it, should only take one stagger. If not do it all over again.
The weapons and gear I use are:
Fang: Dragoons lance to Relic x2 upgraded Royal neck for 40% phy/mag def, one STR and Gen gloves.
Light: Gladus to Relic, x2 upgraded Royal neck for 40% phy/mag def, one STR and Gen gloves.
Sazh: Ant. Deluxe to Relic, x2 upgraded Royal neck for 40% phy/mag def and both Rare / Common treasure hunter items.
I cannot stress on how much Slow rapes these things, the bigger ones are just as easy with slow just follow the same steps, but land the slow on its body and not legs.
With this set up I have the small ones down to around 3 mins and the big ones to 10-15min depending one what they spam.