what'd i do wrong?

[MateriaZ]

so ive had this game for only a couple days. i was doing my second story line dungeon when i was cussed out by a white mage for not tanking "properly". understand that being the tanker means i take all the hits for the party, but every so often, a creature would break off of me and attack the white mage ONLY. id stop what im doing and shield bash it or flash and itd lock back on to me. eventually, he stops healing me and i end up dying, followed by the rest of them. am i still at fault? if it is, what can i do to prevent it? i use flash every 5 to 6 hits and i link my combos. do i have to target the creature i want with flash or does it just work for all creatures around me? im a bit new to this enmity thing.