How to level up at Endgame on Final Fantasy VII

I have already been through this area and I am on Disc 3, however I do not have enough HP/AP to defeat the Ruby and Emerald Weapon's, which I am wanting to do before going to face Sephiroth. I found an answer to getting those up quickly on this site:

The best place to level up in ff7 is the Swamp area in the Northern Crater, it is in the final dungeon of disc 3, but is by far the best place to level. If you are leveling to fight the optional weapons (Ruby, Emerald, Ultima)

I have already defeated the Ultima Weapon funnily enough, but the other two just wipe me out before I even get a chance to make a move, I have tried going back to the Northern crater... However when I try to go past the shack where the old man first took us before we make the climb up, he keeps coming out telling us to just forget it and I cant get any further.

Does anyone know how I can get back in there to level up? Or any other methods of quickly leveling up my Characters?

• For strong enemies, you have the Sunken Gelnika that you can access with the submarine. You can also use triple growth equipment here to level up you Materia as well (Remember that if you get Materia to Max level it will generate a new one, allowing you to get Final Attack Materia for everyone, duplicating Knights of the Round summon or selling All Materia for 1,4M). Also, the enemies in the Gelnika can be Morphed to get items that increase your stats permanently.

• In the final area, taking second fork to the right, there is a grassy area with Magic Pots. They give a ton of XP/AP if you give them Elixir. However, you need to "cheat" with the W-Elem trick or use all Elixirs you can find.

As an advice, to survive the strongest attacks of the Arma Weapons, Final Attack Materia linked with the Phoenix summon is your friend. This Materia triggers the linked magic when you die, so you will be resurrected. By maxing both of them, you will get duplicates so you can equip this combo to all the members of your party.