If you're a PLD and want to stay as one, I would honestly advise you to master switching targets.

Flash will only work if you have a headstart. If your BLM gets the agro, then flash would only get the agro for half second before the BLM gets it back with a Fire.

Therefore, the only way is to combo Rage of Halone on that enemy then flash again. Would fix it but the BLM will probably get agro again if you don't Rage of Halone on it again.

Switch targets. If your dps is killing another mob than you then he's bound to get agro if the fight's long enough. Switch target to it, Rage of Halone combo, then switch back again.