You succeeded where I failed. Maybe you can show this failure how it's done.
Cor is strong and swift. He starts the fight with two wide horizontal slashes. The first one can be parried, the second one cannot. After this, he will fight regularly while occasionally using his own teamwork attacks.
His attacks can be blocked when he is not glowing red, and blocking in the last moment opens up opportunities to parry. Successful parries put Cor in Vulnerable state, during which he takes increased damage and doesn't attack. Blocking and parrying Cor's normal attacks boosts the rage meter, increasing Gladiolus's damage output.
When Cor glows red, his attacks will shatter Gladiolus's shield and can easily kill him in one hit. Cor has various attacks at his disposal, but his most devastating one is where he draws out his katana from its sheath for a powerful slash. He also throws small shuriken for ranged damage. He can be hard to hit as Cor dodgerolls and Gladiolus's attacks are rather slow. When he is in low HP he glows red consistently with only small windows without it, so if the player can land a good parry and Glaive Art on a Vulnerable Cor when he has around half of his health left, they can skip through some of the harder parts of the battle.
Parrying Cor's attacks is the best way to take him down. The player should still aim to block Cor's attacks to avoid damage when he is not red, but parries are superior at building rage. After blocking (not parrying) the player can press forward and the attack button to perform a lunging strike that is fast and almost sure to hit, although it doesn't do huge damage.
The player should try to trigger a parry by guarding at the last second. The player should not parry amid a melee combo because Cor will dodge it. The player should try to raise the shield on the first attack of a melee combo, as this parry will hit and make Cor Vulnerable. When Cor is throwing shuriken the player can parry no matter the distance. When Cor is throwing, the player should try get close to him and parry, as Cor won't dodge.
When the rage meter is 3x or 4x Gladiolus should use a Glaive Art. Pulling off a parry can be rare, so the player should simply use any Art they have rather than wait for Maelstorm. A Vulnerable Cor after a parry is the perfect time to use them, but as the rage drops fast from 4x it may be good to unleash an Art then no matter the battle phase to make use of it.
When Cor has a red glow, it is best to stay away and not engage him. If Cor gets close, the player needs to dodge rather than use the shield.