I happened to be pretty underleveled when I made it to Necron, so this is what I did.

My party was Amarant, Eiko, Quina, and Freya

First, as others have mentioned, make sure all of your abilities that prevent status effects are enabled.

Second, I used Amarant so that I could Aura anyone at anytime to ensure I had autolife at all times to prevent a total wash out of my characters.

Third, I used Quina so I could have Mighty Gaurd at all times for my characters.

Fourth, be an obsessive healer for both HP and Status effects. Eiko was used for this in that she probably healed 2/3 of the time and the 1/3 was spent summoning Madeen

Now, this method isn't really advised, it's just how I beat Necron with an underleveled party. I think all my people were in the low 50's. In fact, I wouldn't advise anyone to beat Necron this way - because the battle took me somewhere around 40 minutes. But that's how I did it