This achievement is largely luck based, the trick is to make sure everything is set up for when you do get lucky. The best place to do this is Tycho's ribs. As well, the higher level you are the better, as you will need to sustain continuous fire from higher level 30+ enemies. I was level 48 when I did it, although anything past 42-43 is fine.
Upon spawning into Tycho's ribs run through the first door and come to the second door. Go through the second door and you'll see a kind of shack to your right where a lost legion eternal will spawnhead over here making sure to shoot your gun into the air a few times on your way to make sure all enemies in the area are focused on you. To the right of this shack there is a small corner which you need to back yourself into. When you're in this area no enemies will be able to melee you, which was useful especially as claptrap who constantly was releasing noava blasts when meleed. The enemies will bunch up in front of you and one of them will be a lost legion eternal. Be careful to shoot just him until you have depleted 25% of his health, causing him to ascend. At this point he evolves into a number of forms, but only one will work for this achievement: etenral instability. Many people say that the eternal magus also works, which it does technically, but it does such a small amount of damage it is very difficult to kill anyone with it.
Basically, if you happen to get an eternal instability you just sit there and hope he takes out 3 guys. Something of note, the lost legion soldiers are killed even when at full health by the explosion, but the guardian enemies (the alien looking ones) are left with a small amount of health, so just before the eternal evolves shoot each one of them a few times to make sure there health is lessened.
If the eternal does not evolve into the instability, quicly kill it(making sure to keep the other enemies alive) and another one will immediately spawn, and simply repeat until you get one to.
If all the eternals spawn or the other enemies die just save and quit and reload, repeating until you get it.