Have someone cast Provoke on Defender X. He will continually use Blast Punch on that character, cutting their remaining HP in half. Have Auron use Mental Break and have Lulu cast level 3 Water spells. The same tactic can be used on the Defender Z enemy.
Have Auron use Armor and Mental Break.Its defense and magic defense will be decreasd.Have your strongest characters attack it.If Lulu has learned Flare,then it could be useful. Also make sure you have a high defense becuause the Defender X can use a move called Haymaker which could take about 2,000-4,000 hit points.