For all multiplayer achievements (social butterfly, well traveled, team player, you are playground games) except for "we meet again" is easily boostable. To start grab a friend then start a private online road trip. it will be just the two of you. pick an event that has at least one playground game each time. burn through the 25 road trips. at that point you will have a good portion of the other achievements done. Then back out and start a private freeroam. invite your friend and head up to the airport. Pick either the fully upgraded alpha 33 or maserati birdcage (barn find). As the host pick the drag race. for the race options pick team yes and rest whatever. each race will take roughly 25 seconds plus load times. repeat until both has achievement. since you are still at airport, pick infected. quickly "tag" other play, another 20-30 sec plus load times. Great thing about doing it this way is you dont have to play with non driving idiots, quickly boosted and get the same amount of xp and cash.