3 Iron Giants and 3 Carbuncles on 3 different sides. At the start of the fight, the Carbuncles will start by using a reflect buff. This will reflect magic skills back to the caster. The buff will last for 5 turns, and can only be cast once. This does not block magic limit burst attacks, only magic based skills.
You can see which skills can be reflected if you see the reflect icon when you want to use skills in battle.
In game, it won't show you if your spell will get reflected or not since you can see the red bar, even when they still have the buff. This can be confusing or cause problems if you forget, but it's better to bring physical attack type units for this fight anyway to avoid this problem.
King Mont is close to the ideal unit to bring for the fight. He can inflict berserk against the Iron Giants, full use out of Iron Giant VC, has area resistance master ability, and more if he uses his TMR buff. He can still end up taking a good amount of damage from Carbuncle's magic attacks, which can end up chaining, so it's really recommended to bring magic resistance VCs, or use Bloody Moon.
Rain is another option to bring as a tank. He's good at tanking magic attacks, and can be geared for more phys defense. However, since Rain's attacks are magic based, he won't really be used for other than just sit there and take the hits. Vinera (Winter) is probably a better option here as the magic tank, but she's a limited unit.
All of the enemies are Light, so it's easier to take them down using Dark units. Behemoth esper has dark attack up, light killer, and demon killer, which works on essentially all of the enemies in the fight. Omega is also another Dark elemental esper, and it has Light killer.
Other options are Fenrir, or Leviathan for esper killer which will work for all enemies. There's also Ifrit for high attack, but only demon killer.
All of the enemies are weak to missile and magic, but since magic can be hard to recommend because of their reflect buff, so the -5% missile resist is somewhat of an option.
If you have Illusory Bell, you can gather up the enemies into the area in the middle, and have Frederika spam Barrage from a safe distance, though it might be better to put the bells on King Mont and make him the damage dealer.
Creysse can apply esper killer buff in an area, and also attack from range.