Add the Plasma Pistol to any of your loadouts and you will be guaranteed an easy 20. When you spot an enemy-occupied Mantis, charge up the Plasma Pistol and shoot the EMP bolt at the Mech. It’s highly suggested that you EMP the Mech from the behind for a safer and more direct boarding attempt. Once the vehicle is downed and recharging, make a quick dash towards the back of the Mantis and hold to board it. The achievement will unlock once you have successfully boarded the Mantis.
*Be sure to follow through with a grenade plant in order to obtain the 'Pump Yer Breaks' achievement.