Crackdown 2

To secure a tactical location/Cel stronghold, locate it on the map, head to the location, and hit the 'back' button to call in the helicopter backup... You'll be told that there's too many Cel in the area and a meter will appear in the top right of your HUD so you can see how many you have to kill before your reinforcements will arrive. On your radar, you'll see all the relevant Cel operatives. Ordinary Cel members won't count towards clearing a stronghold - only the ones on your radar.

Some Cel strongholds have more than one tactical location, if that's the case, you'll need to clear out the remaining locations (it'll tell you at the end of each one how many are left) Once you've cleared all the locations for the stronghold, pop goes the chievo!

While searching Pacific City, the announcer will tell you if you have stumbled upon a Cell Stronghold. When this happens two other Cell controlled areas will appear on the mini-map. Go to each of these locations and clear them out (kill a certain number of enemies). Once both areas are secured, a notice saying "STRONGHOLD COMPLETE" or something similar will pop up as well as the achievement.

the below video explains everything that you need to know to be able to get the achievement, you should really be able to obtain it through natural progression of the game.