I tend to find the word "achievement" a misleading term, as if Enlightenment is an accomplishment of effort to attain something outside of us. I'm of the mind that Enlightenment is our natural condition at the core of who we are, and it's really much more a matter of pulling back the drawn curtains to see that Light that has always been there. So the effort, the work involved, is to make no effort at all, to simply get out of the way and let it be. We try to not try, in other words, which is different of course than apathy. And that's far more difficult than one might think!
The reason I and others make this claim that Enlightenment is our precondition, is because you can have those who have had zero spiritual training, no meditation work, no years of discipline, and so forth, suddenly have a full out Satori experience spontaneously. I am one of those people who had such an experience at 18 years of age. However, that does not mean I was now "Enlightened" in the sense that I lived every waking moment in that state thereafter. Hardly! It forever changed my understanding of Reality, and the years following have been all about the long road "home" to that again.
And that is where the 'effort' comes in. We have to learn to unlearn, to seek to not seek, to simply "allow" that which is always already fully there in us to shine through to the conscious, waking mind. We don't "achieve" it anymore than we "achieve" our lungs. We simply learn how to use them better, to be aware of them, and let them fully breathe us. And that is why Enlightenment is not an accomplishment to boast of, it's just simply our natural state we come home to and rest in with boundless joy. Another way to put this, everyone is already Enlightened, but they're just not enlightened to the fact of that yet.