First of all you must have the highwind for this guide.

 You may ask :

Where do i have to train? Or what materia do i need? What about equpiment?

Well, I'll give you the answers

Training Areas

The red cirle(picture) and the northern crater against dark dragons.  You can only do the northern crater method in Disc 3, it is A LOT faster than the disc two method

The Materia

Max Cover(lower level covers wont always work)

Max Hp plus(recommended)

If you have counter attack un-equip it

The Equpment and Items

Ribbon(accesory)

The monsters in the first method can confuse you, resulting in death and loss of limit build up.  X-Poitions or Elixers are needed.  It is CRUCIAL to use a hyper on your selected character to put him/her in fury so your limit bar goes up twice the speed.

Guide

In the first method look for the monsters that resemble like lions(Bandersnatch - try to get them in groups of 3 so they attack faster) and bunnies (Jumping  2 of them works best)

Say i only had Red XIII's level 2/1 limit Blood Fang.  I would equip him with cover and hp plus and ribbon.  Go to the jumpings and bandersnatchs and just sit there while they attack.  Your character equipped with cover will take all damage(with fury, so x2 the limit build).  This will get your limit to full in about 1-2 minutes.  So approximately 15 mins to get to Stardust Ray(2/2 limit) Tip: If below 3000 hp use an x-potion.

The Dark dragon method is a lot faster because it usually hits 1.7k damage and you just need to heal and use the limits. 

I haven't edited this guide or looked for gramatical errors because of time.  If you see any errors, please write it in the comments.

This is my first guide, so don't write hate comments and such.