This stronger version of Ifrit will mainly use two kinds of attacks: the powerful Explosive Flame Shot and the unavoidable Hellfire. The Hellfire will roughly deplete 30,000 to 40,000 points of Zack's HP, depending on his current Spirit stats.
However, if Zack is equipped with Genji Armor, this should not be too much of a problem as the armor will heal 6,000 points of Zack's HP every six seconds. The Genji Shield is also useful because it regenerates Zack's health if it uses Hellfire. Keep moving around the Ifrit while casting Graviga on it until Ifrit's HP is below 99,999 points.
After that, hit it hard with Costly Punch. The player could also use the Blaze Armlet, as this absorbs damage done by Fire-based attacks. This will render Hellfire and his Explosive Flame Shot useless.