EDIT: This achievement has been discontinued since August 2019 when the servers shut down. Sorry.
If you can, use seven friends to boost this. Otherwise...
You can create and use your own race courses when hosting online events. Make an unusual one that greatly rewards you for using the best shortcuts, and keep racing on that course. Your familiarity should give you the edge.
Also, to be pedantic, you don't need to win the first 8-player online race you attempt. You just need to win one.
Capt Poopey adds some useful advice:
"If you're organising a boosting session for this achievement, make sure that all boosters have downloaded the massive 1GB update beforehand otherwise those without it cannot join the host's freeburn session. Mainly applies to gamers who have just started on the game. Takes about an hour to do all 8 races, and if people don't have headsets or don't want to talk just message everyone in the party that the races will be done in alphabetical order to avoid the wrong person getting the win or the same person getting it twice."