I just beat him a couple of hours ago after him bitch slapping me around the place 5 or 6 times. The 2 key roles to use in beating him are the Sentinel & the Saboteur. With the Sentinel's Provoke, Challenge & Mediguard abilities Cid will focus all his attacking on your Sentinel all you gotta do is heal them & keep them alive.

Then the Saboteur is also very important. The abilities Imperil, Slow, Poison, Deprotect & Deshell are very handy for evening the odds a little but the most important of all is Dispel given Cid casts Haste, Protect, Shell & Vigilance on himself & if you can't get rid of those buffs he becomes very hard to kill.

After that you gotta just hit him with everything you got to get that stagger bar filled!

The medic, commando, medic paradigm is also very helpful to heal fast & to keep the stagger meter up whilst he is in Guard.