The three "Road Kill" achievements are cumulative both through the Gaea campaign and the strategy game. To run over an enemy infantry, you must be controlling a Walrus and steer it such that you run over one of the robots the enemy AI uses as foot soldiers. Any infantry killed by gunfire, your carrier, turrets, or vehicles that you are not directly controlling do not count for these achievements.
I would suggest doing this on the first island you invade after commandeering your carrier (Arachnid) as generally you will want to shoot the infantry from afar to reduce damage you take. In your first Walrus after heading up the hill from the beach, the road turns to the right. Around this corner is a turret, and to the right of the turret is a group of three infantry. Save as you come around the corner, run over the infantry, and then load your save. Rinse and repeat until you have 100 kills, at which point the achievement will unlock.