I guess it is all dependent on your group.
I believe we were myself as Tank and along with a Mage, Archer, and do not recall what other classes.
I could not remember it all to well as we finished fairly quickly. I believe when the Nail dropped someone in the group said to focus on it and i do recall the only one attacking from up close.
If a tank gets enough agro on Ifrit i would say for everyone to focus on the nail, tank included as if it is not taking care of i heard Ifrit's next attack can wipe everyone
I know at first going into it i thought for it to be daunting as i am still new to MMO's but it went great actually.
Also true about gear, always best to have the same level or higher as the Dungeon and or Quest