In Buried, Pack-a-Punch a weapon during round 1 with no one using the bank or Weapon Locker.
There are three ways to do this trophy, but I will only go into detail for one method. I will however provide videos for the alternate methods.
Leeroy Charge Method:
The best method for doing this is the Leeroy Charge method. First of all, jump down the shaft and grab the Remington 870 MCS chalk off the wall, then apply it to the ? for 1,000 points. Head into the Sheriff's Office and let Leeroy out of his cell, then break the first barrier with the booze. Head over to the Saloon and open the door for 1,250 points, then grab the second bottle of booze and use it to break any of the barriers from as far away as possible. This is key to the method: the farther away from a barrier Leeroy is when you give him booze, the more points you will get when he destroys it. You can get over 2,000 points for some of the barriers if done correctly. For the sake of this trophy, you simply want to earn anything over 1,000 points per barrier.
Knowing this strategy, simply buy booze from the register in the Saloon for 1,000 points a piece and break open every required barrier. Once you break open the Gunsmith, grab the weapon chalks and place them on the five ? around the map, which can be found:
Outside Sheriff's Office
At the top of the stairs of the Sheriff's Office
Upstairs in the Saloon, on the balcony (accessable through caves via the Barn)
Upstairs in the Courthouse (requires 1,250 door)
Upstairs in Church
Every time you apply a weapon chalk, you'll receive 1,000 points as well as a bonus 1,000 for applying them all, for a total of 6,000 extra points. When you place the chalk at the Saloon, make sure to jump over to the Sweets Shop and turn on the power! Without power, you can't use the Pack-a-Punch Machine. Combining the chalk points with all the points you will have accumulated from the barriers, you'll have more than enough for a wall weapon, the door to the Mansion and for Pack-a-Punching.
Buy a decent wall weapon, as you will need one to get through the Mansion (PDW-57 or B23R). With at least 6,250 points in your posession, open the door to the Mansion. Now for the hard part - if the Mistresses inside the Mansion hit you, you'll lose 2,000 points and ruin your chances of earning the trophy. Head through the Mansion and kill all Mistresses before entering the Hedge Maze. Go through the Hedge Maze until you reach the Gazebo, then head down the spiral staircase to find the Pack-a-Punch Machine. If you're still on Round 1 when you Pack-a-Punch your weapon, you'll earn yourself this trophy.
Below is a video demonstrating how to do this, courtesy of GxdlikeHD.
Other methods can be found below:
Another method is simply acquiring the Paralyzer from the mystery box and using it to fly over the barriers to get the weapon chalks and their respective points.
Below is a video demonstrating how to do this, courtesy of Rooster Teeth.
Trample Steam Method:
Alternatively, you can simply create a Trample Steam if you don't want to rely on luck with the Mystery Box. All parts for the Trample Steam can be located in the General Store.
Below is a video demonstrating how to do this, courtesy of S1ipperyJim.