Necron is the only bad point about the game, every other aspect of it makes FFIX an amazingly superb game, but they spoiled it by putting a number and mediocre boss as the last battle. Trance Kuja should have been the last boss and he should have been 10 times stronger in all areas because Trance Kuja is just no problem. he should have had 500,000 HP and had moves that take off 7000, it would make it a hell of a lot more challenging and you also wouldn’t get some random bastard popping up as the final boss.
So therefore, as long as your party is above level 45 (preferably 50) with status and elemental defenses, then he’s a joke. The sole attack that you really have to watch out for is Grand Cross. It does no physical damage but instead hits all four characters with several random and very nasty negative status effects. Most you can protect yourself against, but the ones which you can’t, like Zombie, Curse and Instant Death, cause the most trouble.
Although it is rare, it is possible for Grand Cross to instantly kill all four characters (it’s happened to me once), which then results in the Game Over screen (unless you’ve used Phoenix once before and have plenty of Phoenix Pinions in stock by which case there is a remote chance that Phoenix will revive your party upon death.). Mini is another one which is annoying, but not really dangerous. So, apart from that, Necron is quite the push-over (especially if your party is at a high level).
Here’s the standard party I use:
Zidane, equipped with the Ultima Weapon, and High Tide (allowing him to use Dyne a lot more quickly)
Garnet, equipped with the Whale Whisker, with Ark learnt along with Boost and Odin’s Sword.
Vivi, equipped with the Mace of Zeus, with Flare and Doomsday learnt, and High Tide (allowing the possible use of Double Black Magic.)
Steiner, equipped with the Ragnarok (if not, Excalibur), therefore able to alternate between using Shock and Flare/Doomsday Sword. Everyone is also equipped with status defenses and Auto-Regen too, assisting with healing.
As long as you keep the heat up on Necron, he shouldn’t give you too much trouble.