Green Day: Rock Band

Read the description carefully: there are THREE parts to this achievement.
1) Complete the challenge (you have to get the required number of stars)
2) Get 100% on any guitar solo in the challenge
3) Get 100% on any harmony vocal part
...and all three have to be done in the same run of the particular challenge.

The vocal part DOES have to be a *harmony* part--getting 100% on a song that has just the single track won't do it. This actually means that on certain challenges you CAN'T get this achievement because none of the songs have harmony parts and/or none of them have guitar solos.

However, you DON'T have to hit any of the harmony x2/x3 bonuses. As long as you get 100% on the vocals (which technically means getting "Awesome" on every phrase), you'll pop the achievement. You can do it with just one microphone.

A good Challenge to use is the first one from the second venue called "City of the Dead". It starts out with "Boulevard of Broken Dreams" which is both a harmony part and contains 2 guitar solos--so you get a second shot if you muff the first one.

You DON'T have to do both parts of the achievement on the same song, but BoBD is a good candidate since it does have both: you'll know for certain at the end of the song whether you've done it or not. (The actual achievement will pop at the completion of the challenge. All signed-in band members will get it, too, including drummer and bass player, if they're playing the challenge.)

And since it is the first song in the challenge, you don't have to replay 2 or 3 other songs again to take another shot at it.

Once you play enough songs in The Warehouse, unlock the challenge 'Like a throbbing tooth'. To unlock challenges, play the first set of songs in the warehouse, then go to the bottom where it says 'unlock challenges'. If you have enough stars, it will be available to unlock.

Once I had that unlocked, I backed out and connected guitar and mics. I played the challenge voxtar style with guitar on medium and 2 mics on stands in front of me on easy. First try I got harmonies on FOD and hit the guitar solos on the other two songs. FOD didn't have a guitar solo. You should only have to hit one of solos. The achievement pops after the whole set is done.

Before doing the above, I had hit the harmonies on the challenge solo, then did the challenge with guitar hitting the solos. This demonstrated that you have to do it voxtar or with a second player singing if you can't play and sing at the same time.

Your progress does not start over when you change instruments or add a second controller for the mics.

I did it by playing No Place Like Home challenge, both parts (voice and guitar) on easy.

The second song (Restless heart syndrome) has a solo and the third (Last Night on Earth) has very easy harmonies.

Basically I was singing and playing the guitar at the same time, and got the 15 stars with relative ease. Anyone who knows the tune of all 3 songs can do it in a spell, as it is only required to hum in tune and time, so attention can be paid to the guitar part and get the 100% solo, and a decent amount of starts on the songs 1 and 2.

Now, for song three, I do recommend to practice singing the two harmonies beforehand. There is a second voice only for 2 phrases, and there is very little overlapping, but to get the 100% on those two phrases some practice and good breath management is required.

We had some problems with the achievement, looking at the above guide so here is what we did.

HE only had one mic, so we did the challenge "no place like home". We learned we actually have to hit all the blue all the orange and all the brown in one song. I nailed the solo in restless heart syndrome, and he did the vocals on LAst night on earth. If you remeber the orange and blue have nearle no overlap in that song so it was pretty easy for him.
ALSO BE CAREFUL: Restless heart syndrome's solo is extremly hard to get 100% on on expert or hard. try lowering the difficulty

Notes
-harmony and solo doesnt have to be on the same song
-You must hit all of each color, not just get one hundred on one color
-You MUST PASS THE CHALLENGE AND GET THE RIGHT AMOUNT OF STARS
-The achievement pops after the last song

play a 3 song challange and have your gamertag on the mic and leave the guitar as player 2 play medium on mic easy on guitar or medium on both/easy on both and you will get it you don't have to 100% both parts on all 3 songs you can 100% each 1 on what ever song of the 3 that you 100% wich part it is you will get it at the end of the challange