Striking First is an achievement in Plants vs. Zombies: Garden Warfare. To earn it, the player must earn the "First Strike" boast, which is done by being the first person to vanquish a player of the opposing team.
One way of earning Striking First is by playing Gardens and Graveyards as the Chomper and waiting around the place where the zombies spawn. Then, when the zombies are let loose, burrow and eat one. You could also do it as the Cactus and plant Potato Mines in front of the zombie's spawn point. This can also be done easily with the Peashooter's Chili Bean Bomb or Sombrero Bean Bomb.