What is the easiest way to beat that dumb Ruby Weapon

[Jolts]

You could also try the same tactic that people are starting to use on Emerald WEAPON.

Get Cloud at full limit with Omnislash. Then, attach 8 Mime+Counter combination (Ultima Weapon and Wizard Bracelet for 16 slots total). As soon as you can (and after Ruby burries his claws), use Omnislash. Every time you get attacked after that, you'll possibly counter it with 8 Omnislashes (some of them may not work, but most of them should, having them MASTERed helps a lot).

This method will involve a lot of time, though, because MASTERing Mime seven times will take a long time, and so will MASTERing seven or eight Counters. The best way to get a lot of AP fast is to fight a lot of Magic Pots or Movers (Magic Pots are worth 1000AP each, and Movers (which always come in threes, and are very easy to beat) are worth 800AP each for a total of 2400 per Mover battle).
If I remember correctly, both of these enemies can be found when you turn right at the first intersection. Also, the Magic Pots can only be killed if you give them an Elixer first. If you know the W-Item item duplication trick, use it. If you don't, and don't want to, then you can steal Elixers from Master Tonberries (little, green creatures with stars above their heads). Master Tonberries can also be morphed into Ribbons, which are useful for protecting you from the Magic Pot's Bad Mouth attack.

---EDIT---
If you make any other action after using Omnislash, Cloud'll mimck that instead, and Ruby will counter KOTR with Ultima every time it's used, unless he's paralyzed by Hades first.