Necron has an annoying habit of casting spells in this order:

Blizzaga
Shell
Protect
Grand Cross
Neutron Ring
Firaga
Thundaga
Blue Shockwave
Blue Shockwave
Grand Cross
Neutron Ring
Thundaga
Shell
Protect
Blizzaga
Firaga
Blue Shockwave
Firaga
Firaga
Grand Cross
Neutron Ring

The thing you have to watch out for is Grand Cross, followed up by a Neutron Ring. Not only will you be affected by lots of s**t, you'll take on 1500 - 3000HP damage. Necron will usually use Blue Shockwave on 1 or 2 characters, which reduces their HP to one. Think of it as the same as Kefka's Fallen One spell (Reduces all char's HP to 1), but only targets one char. He'll use something that targets all your characters, killing at least one char. Here is a useful party:

NON-GAMESHARK:

Eiko - Make sure she knows Phoenix and Curaga
Amarant - AURA HELPS HERE!
Steiner - (If he has the Exaclibur II, not even the Ragnarok. If he dosen't have the Excalibur II, use Vivi)
Zidane - ONLY if he has the Ultima Weapon. If not, Use Quina. This is why you need Steiner's Excalibur II, because Zidane may not have the Ultima Weapon. The Excalibur II does more damage, but just a bit.

GAMESHARK:

Beatrix - Always do Shock and Climhazzard, use her to heal too.
Zidane - Ultima Weapon.
Vivi - Flare and Doomsday
Dagger - Adiitional Healing and Eidolons