Spike is an ability you can buy from Opher the ?monkey? in Wellspring Glades. To get this achievement I would highly recommend upgrading it to lvl 2 which allows the spike to explode when thrown. I'm not even sure if it's possible to kill multiple enemies without the upgrade.
The easiest enemies to get this on are the bees coming out of beehives. The first hive I came across was in Baur's Reach directly above Baur the bear. Once you enter the Reach head right and then up in the same large area, then left across the expanse to find the hive. Wait for a few of them to spawn then throw the Spike and it'll explode and kill a few. Yahtzee.