Here's what I had before going into battle against him;

Auto-Haste on the four characters I used (Zidane, Dagger, Freya, Steiner)

Status Abilities Equipped (Jelly, Locomotion, Body Temp, Antibody, Clear Headed, Loudmouth, stuff like that.) The only things you can't block against are Doom, Mini, Zombie, and instant Death.

I equipped two characters with Corals Rings, and the other two with Maiden Rings, and had Zidane wear a hat that nullified Ice Attacks, and had Dagger wear a hat that nullified Thunder Attacks, meaning Zidane had a Coral Ring, and Dagger had a Maiden Ring.

Here's what I did to kill him;

I had Zidane equipped with Bird Killer, and did Normal Attacks, which ranged from 6,000-7,000 damage when Necron didn't have Protect.

I had Freya use Dragon's Crest every turn since it did 9,999 damage no matter what.

I had Steiner use Shock, which ranged from 5,000 (when Necron was under Protect) to 9,999 damage.

I had Dagger on standby to heal and use Magic Tags/Remedies whenever he used Grand Cross.

Took him out on my first try.