How to catch a Chocobo

[Sephiroth]

You don't actually need a chocobo to cross the marsh, simply start crossing, when the Zolom gets near open the menu and save the game, then reload, each time you save/reload it regenerates the Zolom at random points in the marsh thus slowly allowing you to cross without a chocobo.

You forogt to tell him not to save and reload but to defeat the Midgar Zolom.