This achievement is remarkably fast!
Start a Strongholds match with these settings:
Game Type: Strongholds
Starting Pop: 60
Additional Pop/Stronghold: 10
Unit Ability Cooldown: 200%
Fog of War: OFF!!
Leader Powers: On
Required: EMPEROR!!(powers recharge 2x fast)
Enemy AI: Easy, Cutter's a good choice.
Select Pavium and start the strongholds on any map you choose, although Sentry will probably work well.
Once the match starts...
1. Go around and build all the Strongholds
2. Set a global rally point nearish the enemy base, in the front of it to try to block the units they'll produce
3. Spam build a bunch of units and make sure to call in Pavium from the War Council..
4. With Pavium, start using his Y ability to target enemies as often as you can, and try to get groups of enemies!
5. Start using the Orbital Designator leader power when it unlocks as soon as you see good groups of enemies. It counts for each enemy you target as far as I can tell! I'm not sure if it counts for buildings as the achievement happened so fast I didn't notice.
If you can test it out and see if it counts on buildings, that'd be great if someone can let me know!
6. Don't forget to build some units to hold the front lines with Pavium.