For this achievement I've found a good way to get like 25.000 - 45.0000 Credits/EXP in about 10 Minutes. I'll try to explain as much as possible, as long my English isn't the best ever.
First of all, While in-game press
Button, Then go to Online Road Trip
just bellow World Map, You'll see 4 Online Game modes. Move
to the right where is located Playground Event
To confirm and wait while the game find a server to you guys play !
Alright, In this game mode, Will be able 2 different events that countain on each map you acess. Infection
mode that will give you almost the same money and EXP that you can get in a normal race on campaing, And the King
mode that is our goal to get more, and more Credits to buy expensive cars to our Garages, in this mode the game will choose randomly a player who will be marked by a crown (The King) , You need to touch the King's car to get his Crown and consequently be the King.
Each Match will take exaclty Four minutes, Except Infection ones that can take less time if everyone gets infected ! A King's event will give you around 25.000 to 45.000 EXP/Credits depending on your performance. Try to use some road skills to improve your reward, so just keep playing it Over and over again !
Doing this method you can get ''Well Travelled''
Achievement. ''You Are Playground Games''
and probably you can get the ''Gold Wristband''
To complete my solution, After you get around 1.500.000 Credits press
in-game then go to Buy Cars
And search for expensive cars to acumulate cars value on your garage... Doing it you'll probably get this achievement. I know it will take some time but it's the only solution I've and I choosed to share with you guys.
Hope it help you guys, Please comment if you have something to add to this guide and leave your opinion about it.