This achievement happened much faster than I expected! I was able to get it after about 5 minutes not long after getting "Freeze". I used a Stronghold's match, spammed Cryo Troopers, and spamming Serena's leader powers.
That included reading and using Serena's units for the first time, exploring her leader powers for the first time, etc, so it could probably be done super fast if you know what you're doing!
Just start a Stronghold's match with Fog of War turned off, and helpful skulls on(in particular, Emperor Skull for power recharge times!)
Choose an easy or medium AI(I chose Cutter on Easy)
Capture all the Strongholds, then on the Stronghold closest to the enemy, build 3 barracks, and spam cryo troopers! Don't forget to put a global rally point closer to the enemy base with
The Cryo Troops will freeze 50 troops in no time, and whenever there's a large crowd frozen, use Seismic Blast to get credit for "Insult Upon Injury". Spam Serena's other Cryo abilities for hopefully more kills.
You'll get a LOT of infantry spammed at you in the first phase of the Stronghold match(3 minutes?), so if you don't get 20 kills by the time the first phase is over and you don't want to deal with freezing more powerful units(I did it with more powerful enemy units), just start another Stronghold and do the same thing, and you'll easily get your 20 that way too against easy infantry.
Very easy, far easier than I was expecting!!