Rock Revolution

Online is pretty much dead for Rock Revolution so I recommend that you grab someone from either TA.com or x360a to help you boost for this achievement.

Once you've got someone just start grinding through songs. I recommend you play ranked matches while doing this so that the two of you can work towards your ranked achievements at the same time as this. After 100 matches the achievement should unlock. After the score screen at the end of each song you'll get kicked back to the menu, so you'll have to host a new match for each song (This is the case in Ranked Matches, at least). Song choice, instrument choice, and difficulty do not matter, but the shortest in song in the game is Blitzkrieg Bop. The person who is losing on purpose doesn't have to bother playing at all and the person who is supposed to get the wins only has to hit one note so they their score is higher than the other person. You can, of course, just play normally with each other and once the better one out of the two of you gets all the achievements the normal way you can start giving the rest of the wins to the less skilled player for free.

Achievements I recommend you work on simultaneously:

Basically what Sebastian said.
But for a significant way to cut time down would be to play Blitzkrieg Bop. This seems to be the shortest song in the game.

It's 2023 and this game is basically dead online, so I opted to complete this achievement using two 360s, an extra dummy XBL Gold account ($1 game pass promo), and two copies of the game. Used ranked versus mode on easy guitar. Connected to the same network I was able to match up the consoles and start a match insanely fast (My reference point was doing the same process for 500 matches in Guitar Hero 3 online, which had abysmal connection speed). Since you can only play one song per match, I chose one of the shortest songs in the game, Stone Cold Crazy. The achievement popped right on the 100th match!

Going for this achievement in 2022, the lobbies will be dead and you will either have to:
a) look for someone who has a copy themselves and is looking to do the achievement or willing to help with said achievement
b) boost the achievement yourself by dual-boxing (using 2 Xbox 360's, 2 copies of Rock Revolution, and either 2 guitars or drum kits)

B is a faster method if you already have all the equipment required (explained below).

If you are boosting this yourself or if only one of you needs the achievements, there is an extra tip of advice I can offer. Once you finish the ranked wins with Blitzkrieg Bop (50 ranked wins - Blitzkrieg Bop is shortest song in the game), you can switch to co-op mode and fail out in the beginning of the song and each fail out counts towards the total amount of online games being played.

Failing out in co-op mode is the fastest way to achieve a play count of 100 total online games. However, since there is another achievement to acquire 50 ranked wins, I recommend doing this after the wins. This will cut the time significantly (first 50 ranked wins took almost 3 hours - last 50 fail outs took 30 minutes). There are less loading / wait times with co-op compared to ranked versus as well. :)