This one's REALLY easy. To "Hot drop" a squad, means to simply have them sent through the Gravity Lift directly to your leader.
So go to "Skirmish" in the Main Menu. Then select:
-Players: 1v1
-Map: Barrens (1v1)
-Mode: Standard
-Difficulty: Easy
Choose the Prophet of Regret for yourself and pick anyone else (for your opponent), it really doesn't matter.
Now once the game starts build a Temple and then two supply pads. Next build up two Halls. Now scroll to the Gravity Lift in the front of your Keep, double click on the Left Stick over it and now whenever units spawn, they will go directly towards this location and be "Hot Dropped" to you. So queue up Jackals to be built, they are the cheapest and easiest to kill with your Prophet's beam. Just build as many as you can until you hit the 40 cap limit, if you want to achieve this one faster, you could also go take out other bases and build Warehouses, it's up to you. Now once you're at your limit, you'll want to use the Prophet's beam on them (poor little guys). To use this, simply point where you want it to hit, make sure you're in range, and press the "Y" button. Kill 10 units and you'll be able to build 10 more. as soon as the last one is trained and comes through the Grav. Lift, you'll have one of the four new Achievements that accompany the new Map Pack for Halo Wars.