In Final Fantasy VII, crossing the marsh without a chocobo is much easier than you think, even though the Midgar Zolom is a big challenge upon first doing so. Take a look at this instructional video and learn how to do so. You do need at least 600 HP to be safe for each of your characters, though 1000 is recommended. It does do about 430 or so damage per action so you'll certainly need to have cure spells at the ready. Plenty of MP is also recommended along with a few phoenix downs in case the snake gets the better of the party. You also must have the "bio" materia.