Beat a level without taking all the plants you could!
Difficulty: Somewhat easy (easy when including VIPs)
Overconfident can be achieved as soon as the player has two shooting plants, like the first level where they can use both the Peashooter and the Aspearagus. It is also easy if the player has purchased a fifth seed slot. Interestingly, beating a level without filling the VIP slot counts for this achievement too, so beating any level without VIPs will work too.