To get this achievement, you have to level up Heller to the point that you are able to earn the Finisher ability. It requires you to destroy five vehicles using this skill. Once you have earned this skill, simply start a few battles with enemies - usually at their bases where tanks and other enemy vehicles will be located - and when you see a tank or APC coming, do as you usually would and mount it. With the Finisher ability enabled however, you can simply press "Y" (for Xbox 360 players) once you've landed on these vehicles to instantly destroy them. Repeat the process, and you should have the skill in no time.