You want to control an AoE (area of effect) unit. A quite common is tanks from the Terran race. Their splash damage is enough to kill either a pack of zerglings or marines rather swiftly. A Helion can also do the trick vs zergling, even better if it has blue-flame upgrade. Tho this is fragile and thus you need to kite the zergligs with it.
You do at least want high ground, meaning you'd want to scout around the opponent(s) base(s) to see when the push out with something you can use. IF the trick still works (been a little while since I played, so not 100% up to date on stuff), you can launch the parasite and burrow at the same time, making your infestor able to stay hidden as the unit moves about.
Good AoE units to look for per race:
- Terran: Tank and Thor
- Protos: High templar and Colossus
- Zerg: Ultralisk
Keep in mind it's only 1 unit, so you want something that can take a pounding as well as deliver, hence helion is really pointless unless there is an enemy zerg on the field as well with lings. Ghosts are no-go. Yes they can drop nukes, but only if one is made, and it's uncertain. The listed units are sure to do the job if used properly. There might be a unit or two I've forgotten, but consider the list the more likely units for you to see.
If possible, what you can do, if you want to cheese it, is to have a friend play something like terran, and you gain control over their tank, before he sends in a small pack of marines for the tank to destroy. However this method may not be viable for this achievement.