Well, I had difficulties with these! Here are some tips to really nail some achievements!
(Secret!) Tail Between your legs- Decline a Boss Battle. (Worth 0gs, but it's nice to have.)
Whammy Mania- Use the Whammy Bar on every held note on Number of the Beast (This one's easy. First, get a piece of tape. Then, tape your whammy bar so that it's help down durring the whole song. Then, go through on easy mode, and his every held note!)
Tone Deaf- Beat any song on the expert difficulty with the games sound options turned down to zero (Not too tough, just go into options, and turn all the dials to zero, then pick an early song on easy mode and blaze through it)
Bronze Streaker/Silver Streaker/Gold Streaker/Streak Master- Earn a 100/250/500/1000 note streak in Career or Quick Play. (This one's a little tricky, but possible. The easiest way to get this one is to get a perfect on Through the Fire and Flames on Easy mode. It has around 1100 notes, so, by perfecting it, You'll get all three. Just concentrate, and make sure you don't miss any notes until you hit the 1000 mark. It's tricky, but shouldn't take more than an hour or so of continuous work, depending on how good you are at the game)
Perfectionist- 100% a song. (Not too tough. Just pick a short, easy song on Easy mode, like Hit Me With Your Best Shot, and get a perfect on it.)
Ready to Rock- Complete the Tutorial. (Easy gamerscore. Just sit through all of the tutorials for this one. It's actually not too bad, especially with the banter between the God of Rock and Lou to keep you distracted.
Track Master/ Got 'em All- Buy all songs/characters from the shop. (Just run through a few difficulties, and you should have enough money for the both of these together. I didn't include Axe Collector, because that's a whole different story, having you beat all difficulties, and then 5 star everything on all difficulties to unlock some of the guitars.)
Leaders of the Pack- 100% a song in a co-op match (Just get a friend who's decent at the game, plop down on the couch to beat a song on easy mode with 100%.)
Two by Four- Get an 8X Multiplyer in a Co-op match. (Same as above, only easier, find a friend, play until you have 4X multiplyer, then activate star power.)
(Secret!) Blowin' it- Fail any song 10 times. (Simple, pick any song, then, when it starts, strum rapidly until you fail it. Repet 9 more times for achievement)
(Secret!) Almost Got It- Fail a song past 90% completion. (Another easy one. Just play a song, and when you know it's about ten seconds away from being over, strum rapidly without selecting a fret until you fail. Five gamerscore coming your way.)
So go out there, lose some friends, and get some gamerscore!
Most people seem to lie upside down to do this and get light headed, but I have another option, and you can play as many songs as you like without feeling light headed... What I did was, got a small mirror and sat it next to my chair and positioned it so I could see my television in it. So it means when I play on lefty flip, you see the screen as normal and can play without feeling light headed...
If you play upside down while trying to beat the game with Lefty Flip activated, I know a way to stop you getting light headed, all you have to do is make sure that you have a drink after each song you play then you won't get light headed. If you still get light headed, just do like 5 songs in 1 session and then play something else.
On the X-Box 360 version of the game...you can buy the Grim Reaper while the PlayStation 2 version cannot get the Grim Reaper...The Grim Reaper comes with the Scythe Guitar and you can use it with different characters.