Ruby Weapon is Time Consuming, yet easy if you know the trick.
Trick 1: Enter with two party members dead. He'll immediately stick the tentacles in the ground and become vulnerable. Revive them quick, however you like.
Trick 2: For emergencies, Final-Attack==Phoenix is always a good idea, 3+stars on each.
Trick 3: You'll want a Mime for each party member--all three. Don't worry about mastering the third, it doesn't matter.
Trick 4: You need Hades at least at 2 stars, and KOTR at least at 2 stars (you really only need one, but for emergencies, a second is always good).
Now to kill him.
Enter, Revive your party fast, then cast W-Summon> Hades THEN KOTR. Hades FIRST.
Why?
Ruby is MASSIVELY VUNERABLE to Hades. It will put him in Stop
100% of the time. Then, the second summon, KOTR, will NOT be countered by Ultima.
Step 2? WAIT. Select Mime on any character, just don't confirm it. As SOON as Ruby starts to wiggle, hit the button fast. Your attack should go off, and stop him again, then KOTR will blast the literal schnit out of him.
Why wait? Well, There is a chance, if you mime immediately, that his stop will wear off just before the end of the KOTR animation. If this happens, you get Ultimaed.
If by chance you do get blasted, don't worry, kill two people, let him make himself vulnerable again, and repeat. Usually takes 8-12 KOTRs, depending on your level and paired materias to the summons.
Just remember, if stop wears off while the KOTR animation is going, you will get blasted by Ultima.