Legionnaire of Doom is an achievement from Plants vs. Zombies: Garden Warfare. To earn it, the player must plant a Doom-shroom, a Bamboo Shoot, a Fire Peashooter and an Ice Peashooter. However, the player does not have to plant them in one session.
Its name is portmanteau of "Legion of Doom", a DC supervillain team and "Legionnaire". The word "Doom" on its name also references the fact Doom-shroom is involved in this achievement.
Legionnaire of Doom is very easy to get, as the player does not have to plant them in one session. The only thing you need to do is get them in sticker packs. Once you have done that, you can play Garden Ops, Taco Bandits, or Gardens & Graveyards, plant them somewhere, and you will earn the achievement. However, on a side note, this achievement can also improve your chances of winning when you plant them, for they are all particularly strong plant pots and can defeat Spawnable Zombies with ease.