Finders Keepers - Hijack 20 enemy units in Ground Battles
For this achievement, you need to play a level in 'Ground battle' mode, where you have an open area and you have to destroy enemy buildings. Later levels of this type let you create buildings of your own. You get a mini-map on your HUD that shows the placement of enemies bases (red) and your own (blue). Many of the Dooku path levels are in this mode.
As you play these types of levels, enemies will drive around in their vehicles and attack you. Attack them back, either in a vehicle of your own but even using a blaster character on foot can work. As their health bar drops eventually they'll be ejected from the vehicle. Now simply run up to it, press Y when prompted to take their vehicle.
Do this 20 times total (doesn't need to be in one level) and the achievement is yours. You could theoretically avoid destroying the enemy buildings that spawn vehicles and get this in one level, or just let it come naturally as you play.