I have had my summoned wasp kill and become a bandit, a bandit leader, and a balverine, and the achievement will not unlock. I've seen numerous people around the internet say the achievement unlocked when they killed a bandit or balverine, yet it doesn't work for me.
"Good men mean well. We just don't always end up doing well." - Isaac Clarke
I didn't get mine until I got the White Balverine.
Without a reason that's mine, one fight is as good as any other. Might as well start with you.
I used a maxed out Summon spell and had my wasp kill a bandit near the Witchwood Cullis Gate. Unlocked, no problem. Not sure the exact requirements are though.
The way I did it, was by letting my summoned wasp kill a hobbe, then let the hobbe kill one of those mechanical minions in the Northern Wastes, then it popped.
I'm sure you can do it much sooner. I just went for that particular one later in the game.