Help to the creation of epic boss battle [closed]

Question

A little intro:
        We are playing in Faerun and currently are in a southern city in calimshan. The story is that this city used to be controlled by a great Efreeti that was banished from the elemental plane of fire, because it was doing business with multiple demons and betrayed fellow efreetis in order to become the pasha of City of Brass. Well this did not work out for him and he was banished in the material plane, conquested a town and after some eons of humanoid slavery, a human slave accomplished to overturn him and free the city of his reign, by expelling him to a demi-plane.
        The human pasha is now dead, and the players find in the city many small portals created by cultists, that summon fire themed outsiders and demons. The players have understood that the cultists have a plan to summon the djinni back, in hope that when he re-conquers the city, they will have some executive position as his assistants. The players are beating the clock in order to destroy the five altars that the cultists are building and prevent the return of the djinni. But I do not think they will make it and I am looking for a way to turn the boss battle in an epic boss fight as it will be the end of a long campaign.
         The problem: is that the (four) PCs are now 12 lvl and they will be at least 13 before facing the boss. An efreeti is only CR 8 and even with HD adjustment will not be a serious challenge for the players. I am looking for something different than just class levels to turn the efreeti into an epic challenge for my players. The best choice would be some demon-themed template applied on the djinni. It will also need protection from cold (so it will not be one-hit KOd by the wizard, but this could be accomplished by items.) and I would like to exploit its enlarge ability and in some way growing it into gargantuan size. I think something that brings the monster to around 15-16 CR will be great, as it will be a solo boss, and thus it must be hard to kill.
         Supplementary info: The encounter will take place in the city's big square. Cultists will perform the final ritual and the square will explode and be replaced with a giant portal on the ground, from where the efreeti will emerge. The players will have a couple of rounds before the whole efreeti comes out of the portal giving the players 2 rounds to either prepare for the battle or attack the cultists and try to slow/cancel the ritual.