This only works if your characters are not yet equipped with armor. Simply buy just one armor for the character you want this to work for and make sure he has no other armor equipped. After purchasing the armor of your choice, save your game, equip the armor, and then load your game. You will notice that your characters defense stays the same as if he has still got the armor equipped but he has not. Repeat the process and there you have it, infinite defense increase.
To easily defeat the white dragon after defeating Brimhild, stay out of its meteor attack range. The dragon will not move. Stay just outside it or it will probably knock out whichever character it hits. It can knock out more than one person. Line Ylenne up on a straight line with the dragon. Then, use Ylenne's Hell Fire attack. It should have the range to hit the dragon if you are just outside the meteor attack. Use the other characters to restore her MP so she can use Hell Fire repeatedly. For this strategy to work, you need lots of Magic Potions and Grilled Eels. The dragon should not kill anyone after you learn the meteor attack range.