this is how i did it
take the charachter with the most hp, for me this was aroun take with yuna (summons=overdrive) and your strongest black mage... (the one with best magic skills)

1. use the move comand to move the black mage and yuna behind the spectural keeper and let aroun (or whoever has the most hp) attack him right in front of him with attack.
2. use black magic and take down his life around 15.000.
3. heal aroun (or the one with the higest hp) with yuna...after 15.000 is taked away start to summon.
4. if you have overdrive, use them!! but if you don't use your best attack.
5. if you have overdrive with your characters to use all of them and just charge him with all you got!!