It's not possible to Mime a Limit Break, so using Ungarmax eight times in a row is not possible, unless you wait for the Limit to charge in the conventional manner before using it.
The simplest way to kill Emerald weapon is to equip KoTR linked to MP Turbo, W-Summon and Mime on your other two (or even all three) characters. This should allow you to get a good chain going.
You might also think about Final Attack liked to Phoenix or Life, with a second Life Materia linked to All.

Regarding the North Crater - You do go there on disc 3, yes, and even though Cloud mentions that there's no going back you can leave whenever you like. Simply run back up and you'll see a hole in the wall of the crater, run through and you'll find yourself back at the Highwind.