What I did was kill two party members beforehand(which is more advisable since it's luck who Ruby Picks, I think)
And then set Cloud up with
Knights of the Round with MP Absorb
W-Summon
And Mime
Basically just KoTR him to hell, your HP will be restored about 8000 HP for each KoTR so you'll be at full, so Ultima isn't really a problem.
