I didn't even realise you could use Lancet on Omega Weapon, first time I beat him...
I was doing ok against him, i.e. I was staying alive, but that sucker just wouldn't die. So I summoned Yojimbo and went all Zanmato on his sorry hide. But then, this was at a time when I didn't have the Break Damage Limit ability, plus I've got the PAL version of FFX - where Omega Weapon's got 999,999 HP, instead of the 'mere' 99,999 that he's got in the other versions.