Freizess has high HP and weak to pierce and fire. There are 2 metal cactuars at the start, and 2 more will spawn after a while.
Freizess has a single target attack that can inflict frostbite, and he always has a buff that increases the chance of inflicting frostbite. If your units are affected by it, your units will stop gaining AP and will take some AP damage at the end of your turns, similar to poison. It can be cleared by using Esuna/Esunaga.
Magic tank is recommended to prevent Freizess from hitting your important units with frostbite.
The metal cactuars might not seem like they would be an issue, but if you are using a dedicated tank, it's recommended that you clear the cactuars to avoid them from hitting your tanks for 1000 damage each time.
If you are lacking some damage, you can try to rely on poison, or dragon killers from espers like Two-Headed Dragon, or Omega.
Freizess is an Ice element magic attacker, and Rain is good at handling against both. He will take less damage because of his element advantage, and he can raise hate by using a TP skill instead of AP, making the Frostbite debuff less of an issue.
Freizess is only weak to pierce and fire, so Oelde can be a good option as the main damage dealer, but he doesn't provide much outside of that.
9S, on the other hand, is able to deal some decent damage and provide a lot for the team. Area offensive and defensive buffs, and lowering the enemy's ATK+MAG. Even better if you have him at EX since he can also lower the enemy's pierce resist, and has a chance to stun the enemy.
If you're using a water team, Glaciela can help with the 3 elemental chain mission. She has Ninja sub job as well to be able to inflict poison to get some additional damage.
Adelard can be a pretty good cheap support for the fight. He's a fire unit, so there's already an elemental advantage, and he can use Green Mage sub job to use Imperil and Deshell/Deprotect. Lowering the enemy's resistances can make it less restricting for which units you can bring to the fight. Yuna is a very good unit for this as well if you managed to get her when she was available.
Shadownlynx is another decent support, but only if you have her at EX 19. She can reduce Freizess' single target resistance by 30 using Shadow Tether, and she can go Dragoon sub job for some pierce attacks, or Ninja sub job to have a move that can inflict poison.
Lilyth has some TP and AP break skills, which can be quite the game changer to prevent Freizess from spamming skills. Same goes for Seymore's Steal Time.