The problem with Ruby WEAPON is that it has a really hard time dealing any threatening damage to you. If you have a Ziedrich, you're looking at maybe 2k-3k damage per attack (15/32 of your HP from one of the tentacles though), which is nothing you can't easily heal, even if you let two party members get sucked in by Whirlsand. Once it runs out of MP it's even less.

And to answer your claim: The Emerald WEAPON Solo Character Enemy Skill Only challenge is considered one of the absolute hardest challenge battles in FF7.