BOSS ANALYSIS : Again, nothing much to be worried about, at least as far as the unknown - 220 Strength makes these brutal. Anyhow, you’ll see a basic Attack and Meteor Strike, the latter of which is magical and Delays. The Attack is only really used as a counter attack in my experience. Of course, you’ll also see Hellfire after the gauge maxes, which is unblockable and seemingly non-elemental. That, or I didn’t have NulBlaze up as I thought I did - thank Yevon for Auto-Life!
MAIN STRATEGY : Auto-Haste is a must in this battle so you can avoid being Delayed every freakin’ turn - without exception, other than Hellfire, you’ll see Meteor Strike every turn. For that matter, Auto-Protect and Auto-Shell (or Shell ASAP and Protect soon thereafter, or an initial Super Mighty G/Hyper Mighty G) also help by halving all but Hellfire. Meteor Strike will still likely hit for 10,000+ damage, so if you have already put on Haste, Protect, and Shell, you’ll want to throw up Ultra NulAll as well - there isn’t any use from the NulAll, but five Cheers and Focuses will boost your Defense and Magic Defense (and Strength and Magic) by 25 points, at least allowing you to survive its attacks. In fact, your Auto-X-Potion will then basically kick in and nullify that damage, leaving only Hellfire, for which you can bring in Aeons as cannon fodder.
So, that’s basically the summation of the battle opening - Hyper Mighty G, Entrust, then Ultra NulAll to get a lot of buffs in quickly. From there on out, Quick Hit is the best way to deal with things since it further helps to counteract the Delay effect of Meteor Strike alongside Haste. If it matters to you, magic is about 6% stronger than physical attacks against Ifrit, so Doublecasting Flare or Ultima also can work out.
Since you’re basically auto-healed with Ifrit’s main offense, it’s simply Quick Hit or Doublecast, Summoning an Aeon as cannon fodder for when Hellfire eventually comes.
ALTERNATE STRATEGY - OVERDRIVE ABUSE : This simple enough. With Break Damage Limit and high enough Strength (or Magic and maybe Trio of 9999 with Lulu), you are able to do massive damage to this boss in a small number of turns. In doing so, you must first enter the battle with as many Overdrive gauges full as possible. Have your main Overdrive user use an Overdrive, then have others Entrust their gauges to that person and keep attacking until done.
Question is, who to use? Tidus has the most reliable one in Blitz Ace, hitting 9 times when done right. Wakka has the luck-based 2x-2x-2x Attack Reels, which hits 12 times. Lulu hits the most if she uses one of her low-level (Fire, Water, etc.) spells with Fury, hitting up to 16 times, but it’s more likely 8 if you have to use Flare or Ultima. Below is a chart detailing the minimum amount of damage you must deal out per hit in a given Overdrive to succeed in this fight for the named number of Overdrives. We only name three Overdrives for several reasons; table width, the ability to hit for under 99,999 damage with these, and stuff like that. Plus, simple division and multiplication can get you to whatever you desire.