What you're talking about is....

SINSPAWN GUI
HP: 12,000

HEAD
HP: 4000

ARMS
HP: 800 respectively

STRATEGY:
This enemy has high defense, and can only be attacked properly once the head and arms are down. So you had better get to that, hadn't you? Use Wakka's ball and Lulu's Fire (or fira) to take down the head first.

Once the head is down, it's time to deal with the torso. But those dang arms are in the way! A quick slash with Auron and Khimari should take down the arms.

Now, with the arms gone, you have a (brief) moment to attack the torso, so make it count! Use overdrives such as Slice and Dice and Grand Summon Ifrit to deal some damage. Once the arms respawn, repeat the process.

Of course, the enemy is going to get a few hits in on you, so use backup skills like Cheer and Protect to lower damage. The enemy attacks with a fearsome Demi, as well as a physical attack, so get healing. Use Power Break to lower the strength of the attacks.

After a while, Sinspawn Gui will bite the dust, only for you having to fight him AGAIN later. But next time you have a distinct advantage.