I don't know much about the shield, but I may be able to help you beat Pylraster.

He is weak against Fire, so you can use Firaga or even Ardor if you have it. You definitely want to equip shields that give good Evasion, such as Crystal Shield. You should equip Bubble Belts to all your characters to increase their HP as well. If you do not have enough Bubble Belts, equip Gauntlets on the characters with shields to increase their chances of blocking. He is also vulnerable to Slow, so use that to get a major advantage. However, when you get him to about half HP, he will use Growing Threat to double his level, causing his attacks to increase in by a few hundred points of damage, increasing his speed, and his Magic Defense will also rise to make it harder to inflict Slow on him. You can equip Nihopaloa (I think I spelled it wrong) and have that character use a Remedy for 100% chance of inflicting him with Slow.

Always remember to keep him on Slow. It will most likely be the thing that wins the battle for you.