just nailed this one a few hours ago, so i thought i would give some advice for what helped me.
usually i turn the mic sensitivity down as it creates an echo sometimes with my television, which is very distracting to myself and any others i'm playing with.
i noticed, however, that turning the sensitvity down meant i had to really belt the songs out leaving little room for error on the harder difficulties (not to mention breathing, lol).
so, rather than turn the sensitivity down, hold down the A button on the controller and turn the volume down, and use X (if you're the only vocalist; X, Y and B are used for each mic) to boost the sensitivity. doing this allowed me to sing the songs at a comfortable level even late at night as my roommates are sleeping.
also, when going for the achievements, if you're still having trouble registering at the beginning of each lyric or phrase, just abandon articulation. where you would never be encouraged to do so anywhere else, just kind of slur the words keeping the correct pitch at priority. you can sing more clearly next time you're singing this song and the achievement has been unlocked. :D
again always go with harmonies, as you can sing a harmony part if the lead is more difficult or vice versa. as long as you get "fab" for the phrase, it doesn't matter which part you sing. just make sure to keep an eye on the correct color and keep with that part through the entire phrase. if i saw a harmony that was one long note, i would just match it and ignore the lead.
keep in mind this may be a little more difficult than it sounds, and may be a little frustrating, as i have a pretty good ear and it still took me a couple tries for boys. so i would say do "do you want to know a secret?" first and get comfortable with almost humming through the part. just make sure your arrow is solid when you're singing your part to ensure that you're getting the multiplier at the end of each phrase.