The Scientist unlocks the Sticky Explody Ball ability at Level 3. You can throw it like a grenade or stick it to walls/floors like a mine to set traps. The Sticky Explody Ball will blow-up with an Plant gets near it. One Sticky Explody Ball will not vanquish a plant so you'll need to use both (you can "carry" two) or soften them up first with your gun.
Progress is tracked over the course of several matches so the trophy will come with playtime. Note that it cannot be completed in Garden Ops (you can't play as a Zombie in Garden Ops).