I got this in my second battle, maybe I was just lucky.
Wait for four weak enemies to be grouped together and then attack them with an alchemist's explosive flask.
The tricky part is finding enemies grouped together. It's probably easiest to find a Cradle that's standing next to a weaker enemy, like a Seed or a Rupture. Kill the CRADLE with a few arrows, when it does, it spawns three seeds. You should have four weak enemies close by now. Now you need a good hit with an explosive flask.
Save your game almost every turn so if you miss or fail to hit one of the enemies, reload. SAVING and trying again usually results in a different outcome (the same attack might deal more or less damage if you just reload...., enemies will act differently etc.)