I was able to get this achievement(in a couple minutes) and "Insult Upon Injury"(5 minutes) really fast by using a Stronghold's match, and spamming Cryo Troopers.
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.
Very easy, far easier than I was expecting!!