Most of the achievements (actually all except for the viral achievement) can be done with only a 2nd profile (gold account required though!), but its way more fun to do it with friends. The online is pretty much dead, so stumbling across a random is nearly impossible. However, I will describe the things the way i did them, step by step.
Now, find a friend who has the viral achievement. Although this might sound hard, nearly everyone has this achievement, so browse through your friends and you certainly will find one. Set up a ranked team game (some say that you need 4 players for a ranked team game but I was able to get it with just me and one other) and win against your partner. (Of course do the same for him to stay fair) Upon winning, you will unlock 2 achievements which are quite self explanatory.
Play an Xbox LIVE multiplayer game against a Banjo team member, or somebody who has already done so
Be on the winning team in a ranked team game
Set up a new ranked game. This time single player and choose a blue print you created yourself. If you don't have one, I'd recommend going to story mode and do exactly that. Now before you cross the finish line, get out of the car and run across the line OR use the reverse drive to cross the line. You get an achievement for every way described, so you will have to do another race to do the opposite. If done correctly, you now should have three new achievements.
Win a ranked solo game with a custom vehicle
Win a ranked solo race by reversing over the finish line
Win a ranked solo race by running or swimming over the finish line
Now we'll set up a premier league game. You will have to play all the 27 games in the league in ONE SITTING, meaning you are not allowed to drop out or do something else. If you have two wired controllers I'd recommend doing it with them, if not you will have to play through it all. Go to match choice, play league with party, and play every race and sport. Gladly you don't have to win, but it's hard enough to just play it. Upon doing that big grind, you've finally unlocked the last multiplayer achievements.
Win 20 Xbox LIVE ranked multiplayer games in any mode
Play in and complete a multiplayer game league
Play in and complete a marathon multiplayer game league
Congrats, you've now completed the MP of the original game!
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