Green Day: Rock Band

As DegenerateGunn's solution suggested,"¿Viva la Gloria? (Little Girl)" is probably the best song to do this one, as it seems to have the longest drum solo segment. But, as suggested by someone on the x360a forums (sorry, I didn't note the user name), if you're struggling with this one, pause the game when you see the drum fill coming. Drop your sticks, and put your fingers over the A-X-Y-B face buttons on your drumset controller. Unpause the game, and just hammer on the face buttons. It should be a piece of cake to get 40 hits in this way. Then switch back to your sticks and finish the song. The achievement will pop at the end of the song. I did this on easy, but difficulty shouldn't really matter, as long as you can complete the song.

If you're still struggling with this, remember that you can track your progress on the in-game achievements menu. Scroll down the list to "Tres Cool," and the progress bar will tell you the max number of hits you've achieved. After my first try, I had 38, so I knew I was almost there.

Thanks also to A Wise Ninja and Stumpy25 for suggesting this be made into its own solution.

Hope that helps someone, it worked for me!

As it says, hit 40 notes in one of the freeform activation drum fills. So, mash away with both sticks on two different pads and keep your foot going. You'll also need to trigger the activation by hitting the green pad (or red on lefty) for it to work.
It will unlock after you finish the song at the results screen.

This is much easier to accomplish on a slower song where the fills are on the screen longer than a fast song with a faster chart scroll speed.

I got it on "Wake Me Up When September Ends" on Expert but I imagine it would be easier on lower difficulties.

I got this by playing Viva La Gloria (Little Girl) on hard. When the drum fill comes, you have a good 5 seconds to whack away. It might take a few tries (took me until the second drum fill) but its manageable.

I happened to get this on Whatsername on the first try, hitting 47 notes. I found in easier when I hit the drum pads with the sides of the sticks, allowing 4 notes (5 notes including the bass pedal) to be hit at once.

I think some of the other solutions might be easier, especially by yourself but my buddy and I used the 2-person method. All you need is a buddy and a second pair of sticks or something similar. When you get close to a fill, ignore the oncoming notes and get ready to spam. I did a drum roll and spammed my foot on the pedal while my buddy did a drum roll on a different pad. If one of you picks the green pad (it's probably easiest for your friend to use the green pad on most songs) it's almost impossible to not hit the crash. We both got it easily using this method.

You have to finish the song for the achievement to unlock.

This achievement may sound hard but it is quite simple in reality.

The easiest thing to do is to try to hit the red and yellow pads with your left drumstick at the same time while you hit the blue and green pads with your right hand while hitting the bass pedal.So you you need to do it hit all of them at the same time 8 times fast and you can do this on any fill.

Not entirely sure if it matters, but I was using the Rock Band 1 drum set.

play 21 guns on expert wait for the drum fill to come pause the game then put your hands on all 4 drum pads/buttons and unpause it soon as the drum fill come continuously tap them real fast you get 10 seconds you need to press all 4 at the same time 10 times and you'll get the achievement

I got this achievement doing "Wake Me Up When September Ends" on Expert. The method that I used is to whack away on the yellow and blue pads with your left and right hands and then I had the double bass pedal going at the same time. I ended up with 47 this way. Using the foot pedals made it much easier. You have to hit the green note at the end for the fill to count and the achievement unlocks at the end of the song.