what is the easy and safest way to get this ach? (the one where you have to kill the wrong mirror image.)
Spread them out, DPS the one you're going to kill down to about 5% and then EVERYONE spreads out away from each other. Then range DPS kill that last one. Almost no one should die.
This is not what we have seen when pushing for the achievement. When you kill a fake NPC, each NPC does a PB AE. The range is somewhere from 75-85. The trick really seems to be making sure your tanks stay alive and pull each mirror to the outside ring and have them all spread out so any one person only takes 1 or 2 hits.
Remove the buff on the wrong reflect you are killing, it only stays down for around 12 seconds so needs to be done when close to death. Put pets on the real one, so the real one dies close to the wrong one. I suspect you are getting a lot of deaths from tanks killing a 2nd wrong one. Be ready for a fresh set of mirrors. We do the first set of the first spawn of mirrors normally and do the second of that set by killing the wrong one. That way if you lose any players you do not have to last too long without everyone alive and full of end and mana and get that phase over quicker.