Learning the whole thing off by heart seems to be a much harder than it needs to be.
Learn the main points of the essay, make sure you know the material, if you know that, you are sorted.
Write it out a few times. Make it shorter each time. Maybe try and write down the start of every sentence.. see if you can remember the rest of the work just from acknowledging the start of the sentence.
Failing that, just writing it all out part by part over and over has always been best for me.