well both is about same' Omega Mark XII, because of it speed and damage each to 4,000-6,000 (when on low health 7,000), it can kill everyone in under min. it took me 15 tries to finally find a way to beat him. and i've beated yiazmat too but it took me long time to beat him, i took me 6 hours to beat him down, when Yiazmat on low health, it combo is higher, one time he just killed everyone under 1 min because he did 7 combo on Fran, then ashe tried arise to fran, yiazmat did 8 combo on ashe - missed the arise, penlno (sp?) tried to arise to fran, yiazmat did 6 combo on penlno, all dead, so i had to switch the party (which is under 30) and run away and only 1 still alive LOL.

I think Omega Mark XII is Hardest snice if u leave and come back, it health will be full again, but Yiamat don't, u can come back and fight with him again untill it death, i've killed yiamat on 8th stage (need more item/MP)