Omega Mark XII is nothing compared to Yiazmat, Omega has only 10 million hp and Yiazmat has 50 million hp, plus Yiazmat uses growing threat which doubles his lvl. So that means he doubles from lvl 73 to 146. It took me 17 hours to kill him, especially after he starts using reflectga on you when he has roughly 4-5 million hp left. Also, make sure you dont use renew while fighting him, if renew gets reflected Yiazmat heals all the way back to 50 million hp again.