About the chocobo breeding part of your inquiry, you won't get to do that before she dies. To go and get everything necessary for the Great Gospel, you'll have to (SPOILER)use the buggy and cross the ocean from Costa del Sol to Junon, then go to the dude's cave to get the Mythril.

And while some people like yourself like to be flawless with their characters, I'll have to agree that it's really not worth it to take the time to build up Aeris if you know she's going to bite the dust.

Regardless, it doesn't matter what we think, though, so if you want to try and do it relatively quick, just (SPOILER)travel to the island where the Temple of the Ancients is located, I believe, and fight the giant birds on the beach. Either the Temple Island or Mideel, I think. Anyway, just put Cover on Aeris and keep fighting those things. Use a Hyper, too, if you think you need it.