The way to farm this is on the first speeder mission in the canyon with Ashoka and Anakin. On the second challenge, where you have to collect the blue point orbs, simply keep replaying this over and over until you reach until you have enough money to buy the x3 multiplier in the store. Then just keep farming this challenge until the achievement pops.
If you can have a second player (either keyboard or WIRED controller) it'll make this go a little faster as they won't try to collect the orbs like the AI partner does.