It depends on your magic and strenght stats, plus if you're using ultimate weapons that's going to affect your limit breaks as well.
Given that Ruby has about 1M HP, at 9k at most per hit in both Omnislash/KOTR the math is as follows
Omnislash = 15 attacks * 9k = 135K per omni => 8 Omnislashes to kill Ruby
KOTR = 13 attacks * 9K = 117K per KOTR => 9 KOTR to kill ruby
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