Cait Sith’s weapon, the HP Shout, can be found in one of the lockers in the Shinra Building on the 63rd Floor. This weapon can only be obtained in the 2nd Midgar Raid so make sure you get it.
A nifty little trick to beat Ruby Weapon without having to fight him is to use Cait Sith. Have Cait Sith on his 2nd Limit Break, and get Cait Sith’s face and you will be awarded with instant victory. This does take some practice but eventually you will get it. This limit break works against any boss (though I haven’t tried on every single boss, but it should work)