Hello all this achievement is first obtainable at the end of the chapter Thug Town Harbor right before The Outskirts.
There will be a platform with two Large barrel throwing zombies. Simply walk to the end and you will see our good friend Rusty hanging on for dear life begging for help. The only way to help him (and get the achievement) is to lob a grenade
on the platform below him. I missed on my first attempt but simply jump back up to the platform and throw again!
I'd label this as easily missed if you just shoot at the zombies on the platform.
TL;DR? Watch me do this and Mechanic Maniac here!