Necron is not as powerful as Ozma, but still should not be underestimated. Necron has a plethora of magic spells, including Firaga, Blizzaga, and Thundaga, which hit for around 1,500+ damage on all the characters. Necron can also cast Holy and Flare, which hit one character for around 3,000 damage. Necron can perform three actions per turn, putting the party at a disadvantage, especially if he casts Grand Cross, since Neutron Ring follows almost immediately after.
Quina's powered up Frog Drop is an easy way to deal damage and Quina can also cast Auto-Life and remove all of the party's status effects with Angel's Snack, as well as buff the party with Mighty Guard. If the party has any Elixirs left in the inventory, this is the time to use them. Necron himself also carries four Elixirs so Zidane can steal them if the party runs out. Freya can avoid the Neutron Ring and Grand Cross attacks with the Jump command, especially if she has also achieved Trance.