Difficulty: Easy
Level: Infiltration of Jackson (Level 7)
Bonus objectives: 1
Time to beat: 7:30
Press the start button to skip the conversation at the start of the mission, then run forward and jump over some sandbags to spawn the first wave of enemies. Grenades are your best friend here. As Smeds recommended in a previous solution, throw a grenade at the farthest building to take out the first and second waves of enemies; kill any remaining enemies with your primary.
Continue through the level to encounter and kill some more enemies, then follow the road to the right, then the left where you will find a Gatling gun. Kill as many enemies as you can with the gun -- I had trouble reaching the enemies on both sides of the gun, so I used my primary to kill them. -- Once the area is clear of all enemies, walk through the yard and press
on each of the 3 barrels to blow up the cannons (the barrels are touchy, so make sure that once you press
the barrel is no longer red before continuing to the next). Once the final cannon explodes, a fence by the left-most buildings will do the same.
Continue through the fence around some corners to find yourself in front of a warehouse with some enemies. Chuck a few grenades if you still have any and make your way into the warehouse. Once you're in, the enemies will stop shooting at you. Run through the warehouse and out the back door to find three more cannons to blow up. Before placing the explosives, run around to the left of the building (or right if you're looking at it from the back fence) and you will see a red barrel. Simply walk into the barrel, then return to the cannons to place the explosives.
Once the final cannon blows up, enemies will start attacking you from both sides of the warehouse, as well as from within. Finding cover at this part is tricky, so what I did was I went around to where we picked up the bonus objective and I killed the 3 enemies there, using the little alley for cover. More enemies spawned, then I killed them and made my way back to where we blew up the cannons. Then I balanced my gunfire between the left and middle of the building. A few more enemies will pop up on the right side too, so just be cautious.
Once the area is clear, the level will end and if you did it under par time, the achievement will pop. I hope this more detailed solution helps out.