This is so wrong and misleading.

That Top 100 black mage from previous seasons with defence buff? He is the huge threat, not that noob ninja with attack buff. Focus black mage first. Yes, you shouldn't affraid that blue defence buff, it reduces your damage only by 10%. No, he is not harder to kill than 13k HP ninja, because 10% extra defence makes his effective HP only 11k, not 20k.

OR you can chase noob ninja and watch that BLM destroy your team with Fire IV spam, the choice is yours.