This cannot be done on EMULATED BASS, but it can still be done with a regular guitar. I got this today and I don't even own a bass guitar.
In the tuner, you need to switch to bass guitar (not emulated). Now just tune the four strings way down low.... you should be able to fool Rocksmith into thinking you have a real bass guitar. This might be a little tricky, because tuning strings that are so loose is so innacurate. But I did it in like a couple of minutes.
Now just do those techniques you might not have hit while playing on emulated.
Go to lessons, pick one that you need and only play through the practice track. Don't quit in the middle of the track, the game needs to save your progress. (At least that's what I think. My achievement popped upon completing -and saving progress- on a bends lesson. I had done bends before but quit out just after a few successful ones.)
I hope this has helped you get another 1000/1000 completion!
Cheers.