It took me many tries to accomplish this for the first time. Then, I started holding position with the medivac and that made the challenge much easier in my opinion. Got this idea from Dakkon.
When the challenge begins, attack with group 1 (all the units). Right when you see that the computer is bringing all his units, run away with group 2 (zerglings). You’ll kill most of his units and it’s gonna be a lot easier to micro.
You might have to target one or two surviving banelings with a couple of speedlings though.