A good way to start off the battle is to use Auron's Armor Break and Wakka's Dark Attack to blind it. If Lulu has learned any level three spells, she can be used to great effect, and the damage she can do will be improved if Auron wipes out Defender X's Magic Defense with Mental Break.
An easy way of defeating Defender X is to have Tidusprovoke it. This way the boss will only use Blast Punch on that character, and since the attack only halves the current HP, the character will not risk being KO'd. This method also applies to the Defender Z enemies found in Zanarkand and the Omega Ruins.
Another way is to have Yuna's summons at max Overdrive gauge before encountering the boss and use the aeons' Overdrives. If the player summons an aeon it should use Overdrive right away, because the Defender X can kill them in one hit.