It's after the islands have been destroyed.

I can't give any advice on the stuff before Necron, but for Necron I suggest all abilities to guard against status effects, and auto-regen.

I've beaten Necron with Zidane having The Orich-summat-summat, Steiner with Defender...that's all I remember. But no, the best weapons aren't neccesary, but they speed it up alot, and look cool