I finished the game back in February and beat Yiazmat to boot! A couple of tips I can give:

Have Windbreakers (you can also try to get yourself and other characters in different positions) to negate the damage to his huge wind-based attack (some characters can avoid the attack depending where they are). You have to be quick, once you see the message that he is about to do the attack, quickly go to the menu and put on Windbreakers and once the attack subsides, put on your previous armor.

The Shell Shield can negate his instant-death attack by around 50% (its the one that has a Pentagram look). Again, once you notice Yiazmat about to cast it, quickly go to the menu and equip the shield. Regardless if it worked or not, quickly go back to the menu and re-equip whatever weapons and shields you had on before.

It took me nearly 10 hours to beat Yiazmat but I bet all of us had a rush of relief and excitement the second we see his death animation!