Have you beaten Ifrit with nobody having sentinel in your party?

[Saomai]

The purpose of sentinel is to have less dps on the spikes for Hellfire, and more output on Ifrit himself, just making the fight shorter. You can compensate for no one having sentinel by dropping additional spikes during this phase. It'll likely extend your time in the instance, but won't require having the ability on macro.

I am sorry, for all the experiences I have, people usually drop one nail. Does that mean that when all nails are destroyed, hellfire won't do as much damage?

[Alerith]

I am sorry, for all the experiences I have, people usually drop one nail. Does that mean that when all nails are destroyed, hellfire won't do as much damage?

Pretty much. Each nail dropped reduces the damage from Hellfire. However, the current, commonly accepted, efficient solution is to drop one nail to reduce the damage to an acceptable level, then use sentinel to mitigate the rest.