Ifrit will always start its first move by using "boil", which increases its fire attack. Ifrit's second move will be an attack, and once it finishes, 4 Grenades will show up on left and right side of the center area. More Grenades will show up as you defeat some of them.
Ifrit's second move can reach all the way to your team's spawn location, and "Hellfire" is a pretty strong fire attack in an area. It's highly recommended to have a magic tank with high fire resist move forward and separate from the team to prevent some damage.
The Grenades often uses fire spells from ranged, but if they are very close to your unit, they will use normal attacks, which can be a problem for your magic tanks. You don't have to defeat all Grenades to beat the fight, so if your team is strong enough then you can keep focusing on Ifrit. It also has high hate, so pressing auto will make the ai focus on Ifrit most of the time.
To clear all the available missions, it will be easier to do it twice. First attempt can be to just beat it without a companion, and the second is with another companion to get no KO. If it's still a struggle, you can revive using visiore to finish the fight. As long as all units are alive when you beat Ifrit, you will get the no KO'd units mission.
Having a magic tank is incredibly important in this fight since most of the attacks are fire magic. While there aren't any water magic tank for the natural fire resistance, you can use VCs like Fenrir,Odin, or even Explosive Orb, Bomb to help with survivability. If you have plenty of water units in your party, you can use Siren for some damage increase.
Both Tidus and Creysse are free units that are pretty good for this fight. Tidus can be a good chainer with his quick hit and burst ability, while Creysse has good range and an esper killer ability from the sniper subjob. She can help snipe the Grenades, or deal more damage to Ifrit from afar. Both of them also have water boosting master ability.
Leviathan esper has +10% fire resist, and its skill board has 25% fire eater, water attack, and esper killer. Definitely the ideal one to bring for the fight.
Fenrir has 25% esper killer, and 15% magic resist. It has negative fire resistance at base, but you can raise it up through the skill board.