ONLY THE ITEM "GRENADE" WORKS. DEO-BOMBS, INCENDIARIES, MOLOTOVS, ETC. DO NOT COUNT.
In Act 2, I found a merchant on the street (protected by barricades) that had one grenade for sale for $600. If you take a look at this map (credit to IGN):http://www.ign.com/wikis/dead-island/Act_2_-_The_City_of_Mor...
The merchant is the red triangle southwest of the church (the yellow pentagon marked with a '1').
I was around level 19 at the time. There are plenty of vehicles in the city to jump on top of, so when you find an Infected (they are the zombies that run and attack extremely fast) leap on top of a vehicle. The zombie cannot hurt you up there, particularly if it is a tall vehicle, like an ambulance. Throw the grenade at the Infected and you should get the achievement.