With a group this isn't difficult at all if you communicate and know what to do.
Prep: So as the description says, do Hoxton Breakout on Overkill with no skills (EDIT 6/26/15: that means no Doctor Bag, Ammo Bag, ECM, Trip Mine or First Aid Kit; those each take 1 skill point and will invalidate the achievement), no armor, and use the two guns they tell you to. What they don't say is that you CAN equip a perk deck, and you can mod the AK and Typewriter as well. For the perk deck, I recommend Rogue, as a maxed out Rogue deck gives you a 30% chance to dodge every bullet shot at you and the Walk-In Closet perk increases ammo pickups to 135%. The AK should be modded for max damage to obliterate all Bulldozers, but other than that you can mod the guns as you see fit.
Day 1: So here's the biggest hurdle. Once you get Hoxtilicious in the truck you need to move it 3 blocks to the parking garage and the route is semi-randomized, with up to three obstacles after the initial cop car blocking the road. There can be (1) a cop car that you have to interact with to move just like the first car, (2) two spike strips on the road that need to be interacted with to remove, (3) road barriers that are lowered by hacking a nearby circuit box, (4) a giant pothole you need to bridge with metal that is in a corner nearby the hole, or (5) an armored SWAT van that needs a 60 second drill before you can move it. The first four take no time at all, the SWAT van can mean death if you get caught with your pants down and the drill is broken, and it usually jams 2-3 times as well, so be prepared to fix it. The pickup truck also needs to be signaled after every obstacle to move again, and will stop halfway down each block even if there isn't an obstacle in the way. I recommend moving ahead of the truck down the route to the garage, as enemies spawn in a radius around the truck, not you. So you can fight them off from one or two directions instead of three or four. On the last stretch one sniper spawns. If your path to walk is past the two ATMs toward the park-ish area, he'll drop in onto the balcony in front of you, otherwise he spawns on top of a SWAT van (not an obstacle van, the ones that act as level barriers to block you off).
First things first, blow away every regular cop you see, since they can spawn with .44 Broncos which will absolutely end you. If you see a regular police cruiser, there are two regualr cops on the map as well. Crouching along the sidewalks is your best bet. Once the truck reaches the garage, grab the parking ticket and rush inside, split up and have each person lockpick a door. Pay attention to the objective indicator as you pick, as it changes right when you pick open the correct door. There is a black Bulldozer with an IZHMA shotgun sitting in there. Light him up with the AK because he actually can't attack you until he completes his animation of getting up from the chair. Start the 30 second hack, guard it and then lower the bollards when it's done. Have one person hide in the room and the other three draw aggro and kite the cops somewhere else if you can. Sprint downstairs and signal the truck again, this time use it for cover as it moves to the ticket machine. One person can hug the front of the car for dear life when it reaches the machine, and take off all 3 sets of armor plating looking through the grill of the truck. Pop the ticket in and you are done.
Ideally you'd be playing with a full crew, and this guide is assuming that, so move in groups of two on the street. Number one tip: never stop moving, there's the ever present if you stop some aimbot cop with a shotgun will drop you right there. If somebody goes down on the street, don't worry too much since there are like 2-4 civilians in the parking garage. Conserve ammo as you go and you should be fine. Day 1 can be the most difficult, but it's pretty simple too.
Day 2: You need to do three things on Day 2, get Hoxton to the server room, help him get the info he needs, and take the server and leave the way you came. While Hoxton is getting his info, you'll alternate between defending the control room and a random objective three times. Each defense wave is 100 seconds if you can protect the power box each time. If the power goes down, there's about a 30 second delay for Hoxton to get up and running again after you turn the power back on. Every door that needs drilling will take 60 seconds.
There are three keycards that randomly spawn that you WILL need for the objectives. They spawn on desks, chairs benches, behind the security kiosks on the first floor, or in the white tray by the entrance metal detectors. One always spawns in the control room, the other two can spawn in any valid location on the first or second floor. The keycards get you into the objective rooms when they activate, the armory (which has what's basically four or so ammo bags and 9 grenades on a table), the infirmiary (which has multiple first aid kits on the wall), and the stairwell to get to the third floor (only reason to go up here is to go close range on the third floor snipers, so it's useless more or less here).
So from level start, there are two guards by the metal detectors, and usually ten in the main lobby area (two behind the counter, four upstairs, four downstairs) although they can move around a bit. The female FBI agents are all carrying .44 Broncos, so watch out. There can be a few civilians here as well, take all of them hostage to delay the assault wave as long as possible (there are also a few in the control room, and one or two by the desks by the forensics lab). Get Hox up to the control room and he'll start doing his thing, use the remaining lull to find all the keycards. You'll probably need to open the armory since your guns eat ammo like it's crack-laced candy. Try to save one card for the final escape run as well. The power box spawns randomly in the control room: that box is home base. Guard it or you'll hate your existence. Once Hoxton hacks the first monitor in the room, you'll get one of five random objectives.
(1) Director Approval: You need to hack the FBI director's computer and then approve 3-4 clearance requests from Hox that come every 10-20 seconds. You simply need to return to the control room after. Send 2 people to this one and leave 2 in the control room as B-Team. You'll cause the cops to divide their attention and make the objective easier. The director's office is on the second floor above the lobby, and you have nearby perfect cover just sitting behind the desk.
(2) DNA Evidence: You need to scan DNA samples in the forensics lab to find the traitor's DNA sample. The lab is just around corner from the control room, on the right side of the building (facing lobby desk as point of reference, same side of building as director's office). You need to drill the door to the evidence room or use a keycard (there's a keycard door in the lab to the evidence room that's more covered if you use a card), and lockpick the lab door. I'd recommend drilling here to save the keycard, as if you sprint as fast as possible there when the objective kicks in, you have at least 20 seconds of free time while they relocate, especially if your team draws aggro somewhere else. There are 6 blue boxes in the bookshelves in evidence marked "CLOWN CASE." You need to scan these, one at a time in the lab, and each scan takes 30 seconds. Once you get a match, just return to Hoxton in the control room.
(3) Encryption Keys: You need to open the door to the security office (either use a keycard or drill) which spawns in a corner of either the lobby or the balconies above the lobby. I'd recommend a card here since there's NO cover at all in any of the spawn locations for the office. Start a 90 second hack on the computer and guard it. There's a power box for the security office in the office itself that also needs to be guarded, and there's a chance they'll flood the office with gas for one minute. Once the hack finished, grab the marked hard drive and then deliver it to Hoxton to complete the objective.
(4) Missing Server: There's a server Hoxton needs that's in the IT department. IT can spawn upstairs next to the director's office or behind the lobby kiosk on the ground floor. There are four offices the server could be in. Shoot the windows out (they can't be melee'd) and you'll see the server on or under the desk in one of the offices. Use a keycard here and get the server to Hoxton and connect it, you'll need the extra time the keycard gives you since the server slows you to a crawl. The three not carrying the server need to escort the carrier back to the control room. After you connect the server in the control room the objectives completes.
(5) Witness Testimonies: Hoxton needs some hard copy files in the archives. The archives are in the basement of the building, blocked by a lockpick door and a drill/keycard door. The drill door is easily defended, so you can drill here if you want, although it's not a bad idea to gain some valuable time searching with the keycard. There are 12 double-sided bookcases in the basement, for 24 search locations total. 1/2 people need to guard the 2/3 people searching the bookcases. Once you have the files you need to deliver them to Hoxton in the control room to complete the objective.
Escape: One person needs to carry the server from the control room to the escape vehicle in the basement accessed next to the metal detectors at the entrance. It's a drill/keycard door. Unless you had to open the infirimiary or couldn't drill a door earlier, you should have a keycard left for this door. Walk across the second floor balcony (like you're going to second floor IT or the director's office), NOT DOWNSTAIRS right away. You can use the pillars as cover and then throw the server over the railing closer to the entryway to avoid walking it all the way downstairs. Wait till it's at least temporarily clear, jump down, pick it up, drop it in the SUV downstairs and enjoy your fresh 90GS!
So the order goes:
1. Defend Control Room
2. Objective 1
4. Objective 2
6. Objective 3
8. Grab server and escape
So, including the escape there are four objectives that can use a keycard (not including the Director's office objective), and you have the armory and infirmiary as optional keycard uses too. 6 doors, 3 cards. Make your choices ahead of time to avoid confusion or argument on the team, because things get hectic and you'll want to have a plan.
-(Day 2 only) Snipers spawn in the lobby area on the third floor balconies and in the entryway after you clear it initially, and outside on the roof across from the windowed hallway on the second floor. They'll drop you in one shot 80% of the time, all the time. Be careful.
-Shields spawn either in pairs or with a taser, if you see one expect at least one more special, if not a second pair moving with them.
-Dozers believe in the non-agression principle between assault waves. If you run into one between waves, DO NOT SHOOT IT AT ALL. It's retreating out of the map and it might despawn if you let it leave, as it WILL fuck you up if you fire at him. Green Dozers have pump action shotguns and Black Dozers have IZHMA shotguns. They have decently long reload animations that leave them vulnerable as well.
Author's Note: I'm American. First floor=ground floor, second floor=your first floor
If there's anything you think I missed, let me know. If you're desperate for a video I can try and run through it again for a guide, although Day 1 is mostly running and hiding, and Day 2 is mostly running and hiding, with some hacking thrown in there too.
EDIT 6/26/15: An Overkill developer confirmed that you don't need a full crew to unlock the achievement, so if anyone drops then you will be fine. You can do it solo if you really want to.