Useful stratigy against ruby? Is that the one in the sand? If so, very easy.
Equip on your most powerful char (usually Cloud):
Ribbon
Try to make sure you have 9999HP/999MP, via HP+ and/or MP+.
Also helps to have a magic+ or two, to make sure you get 9999HP per absorb.
KOTOR (preferably at master, just in case)
Equip it with Absorb HP in a linked slot.
Make sure to have mime equiped.
Enemy skill w/ Big Guard.
Go into a seperate battle before Ruby, kill off the two party members you don't need (since he'll shoot them out of the battle anyway).
Go into battle with Ruby, and start off by using Big Guard. By this time he should have dug his claws into the ground (good thing), which they should be behind you. Summon KOTOR against the two hands, and Mime the rest (absorbing, what likely will be, 9999 HP per summon), making sure to attack the hands (since the main body should be invincible). You'll be able to attack the hands and the body alternately (via Mime), over and over. The battle shouldn't last more than a few minutes.
Even easier stratigy (though it's mostly just a repeat):
KOTOR X2 (master summon + at least 1 level 1 KOTOR summon, different linked slots)
Equip absorb HP to the Master KOTOR, absorb MP to the weaker one.
A reminder: some MP+'s and HP+'s wouldn't hurt if you're not too high of a level, and Magic+'s are always good.
