This achievement is easiest on normal difficulty, be sure you know which hangar is open beforehand (you'll see light coming out of it) so you know which way to go. kill all the gangsters.
You'll need to steal all of the weapons that are in the hangars, including the ones in the trucks, the faster you move with the bags, the less cops you'll have to deal with. There can be up to 12 weapons to secure so make sure you check all the hangers thoroughly.
- Endurance(Aced), Sprinter(Aced), Run and Gun(Basic), Transporter(Aced)
You can open the trucks faster with these methods:
-Upgraded drilling skills (Hardware Expert[Basic] and Drill Sawgeant[Aced])
-Shape charges (Requires the 'Shaped Charge' skill)
-Using a saw (Requires 'Portable Saw' skill)