Rock Band 2

Do a long song on easy and just blow your overdrive whenever it is available. Good songs to choose from are Hungry Like The Wolf and Everlong. There are plenty of other choices though (Aqualung, Visions, etc).

If you're looking for a quick way to get this achievement and don't want the stress of actually having to sing, once you unlock Abnormality's song "Visions", you can earn the achievement here. Just hold the mic to the tv whilst the song is playing and be ready to jump in to activate overdrive.

Straight after the first 2 overdrive phases (when you can first activate overdrive) there's a small window to activate in, but the achievement can still be done if you don't hit this.

The only other part for concern is the first breakdown where the there's a long grunt then a huge silence. There's a large silence where, if you have it, you can activate overdrive. If you activate it too late, you can possibly pick up the next overdrive phase and extend your overdrive.

That said, even if you fail both above parts (I did), you can still get the achievement by activating as soon as possible. Just make sure to have your mic close enough to your tv for it to pick up the talkies and the achievement should be yours.

This can also be a good way to get the 100% vocals achievement if you haven't got it already, however, you're unlikely to get a gold star with the path you'll take.

I found the best song to get this on was Let There Be Rock. Although this song doesn't involve a lot of singing, it gives you the opportunity to rock out at least twice for every lengthy section of singing in the song. I put the song on easy and rocked out as soon as possible every time I was given a star power phrase and I ended up deploying star power at least 6 times.
However, this song is not the best for Flawless Singing, for it's not the easiest, just a good song to provide a lot of star power.

Testify by Rage Against the Machine - if you watch the overdrive carefully you can just barely get the 4th overdrive and because the song is all talky there is no singing needed.

I got this on "Let There Be Rock" by AC/DC on easy vocals.

NOTE: The song I used is the 1977 studio recording (with Bon Scott - tier 4 vocals) found on Rock Band 2 disc, NOT the 1992 live recording (with Brian Johnson - tier 5 vocals) found on the exportable AC/DC Live: Rock Band Track Pack. Be careful of which song you choose if you have exported the songs from AC/DC Live: Rock Band Track Pack.

At one point in the song, there is an area where you'll be able to deploy overdrive just before a percussion section starts. If you miss your chance to deploy overdrive, don't worry about it. When the percussion section ends, you'll get another chance to deploy overdrive long before you need to sing again.

Lastly... set the difficulty to EASY and ALWAYS DEPLOY OVERDRIVE EARLY! Don't worry about optimal paths for deploying overdrive to maximize your score.

I can't carry a tune to save my life, but I did manage to unlock this achievement (using the song mentioned above) with 94% phrases hit on my second try. I even messed up obtaining the last overdrive near the end of the song, though I had already deployed overdrive 4 times before that.