I hated this achievement because no one has this game, no one plays online, I wasn't having any friends over before I had to return it, and my sweetie has no interest in music games, so I had to find a way to do it by myself.
It's really easy if you have four mics and four controllers (which I did after buying Lips). [Edit: 4 heasets will also work, or some combination of heasets and mics -- thanks t4nd4r!] Start a band with four vocalists, all of them set to beginner. Put the controllers in front of you rotated 90 degrees from each other in a circle so that the button groups are all next to each other. You can now either hold the mics in one hand and sing/blow into them, or set them in front of a fan, but the key is to have at least one hand free. The sure fire way is to put them in front of a fan, but you don't necessarily have to -- it'll just require a lot more concentration if you don't.
Now, pick a song that has (1) enough star power phrases at the beginning and (2) at least one band moment. "ABC" is the one I used, although if you're actually singing it may be more difficult. If you're using a fan you don't have to worry at all.
(not all songs work -- for example, "Oh, Pretty Woman" doesn't seem to have band moments and "Happy Together" doesn't have enough star power, from what I can tell)
Now position your hand or hands above the controller buttons so you can hit "a" on all of them at the same time, just wait until you have enough star power built up to launch it, and when you see a "band moment" section (there are flames under the vocal track), and launch all four star powers in the middle of that section.
(remember that you need to have maintained a 4x multiplier up until this point, which should have been automatic if you were using a fan.)
As long as you have 4x on all players at the moment the band moment ends, you'll have your 11x on the band and the achievement will pop.
Anyways, hope this helps somebody out!