There's a total of 4 bosses in Operation Dark Hours, and most are challenging purely because of the amount of things you have to deal with, rather than them being complex. While they're all DPS checks (most have time limits), as long as you're efficient with the mechanics, they can all be farmed in about 60 minutes.

Boss 1: Max ‘Boomer’ Bailey

Max Boomer Bailey will appear after destroying the ‘Signal Sources’ that make enemies immune. Once you destroy them all, Boomer will begin to beat his way into the room. There's several things to be aware of:

• You can only damage Boomer once he is weakened from mini-gun fire on his backpack. 
• Boomer will heal after taking damage, when he gets back up from his knees. You will need to destroy the healing pack on his chest (it'll glow green). 
• The Defence Systems will spawn on rotation and can be destroyed by the mini-gun, or deactivated using the central laptops. The quickest means of dealing with them, is to deactivate them (it ensures your mini-gun isn't overheated when Boomer rolls round). 
• Enemies will spawn, and will need to be regularly cleaned (especially the medics).
• Boomer will target one player, with that player marked by a red eye. When under his gaze, he will only pursue that player.
• Boomer has a berserk timer which effectively sees him destroy both mini-guns after a period of time (around 6 minutes). You should ensure he's dead before he goes on this rampage. 

Tactics 

A fairly easy fight that is quick to do, as long as you can take 2 bars of his armor every time he's weakened.

• Assign two players as mini-gunners, who are responsible for manning one of them when Boomer is in front of either. If either of these players are under his gaze, the other should take over mini-gun duties. 
• Have the rest of your team (6 players) stay together, and move with the player that's being focused by Boomer. 
• The focused player should move clockwise around the play space, leading Boomer into a mini-gun (with his back to it) so that he can be weakened, and then damaged by the 7 players (the 8th will continue to shoot Boomer with the mini-gun). 
• The mini-gunner should always ensure that it doesn't overheat when damaging Boomer's backpack, so he can continue to fire.
• The focused player should always ensure he kills Boomer's healing pack the moment he stands up (it's easiest for them, as they command his attention). 
• Have the 6 roaming players regularly kill the enemies while they wait for Boomer to be weakened, otherwise you'll quickly be overwhelmed. 
• When the Defence Systems begin to activate, have someone on the team move to the laptops and turn them off, to ensure your mini-guns are free to fire. 
• Repeat the process of leading Boomer into a mini-gun > destroying his backpack > DPSing him > killing his health pack > moving to the next minigun until he's dead. 

Boss 2: Ben ‘Dizzy’ Carter & Carl ‘Ricochet’ Dawson

In terms of difficulty, it's not too hard in a pug setting, but Dizzy and Ricochet do need a fair amount of vocal coordination across two sites. 

• Each side of the plane has 3 control panels. Only one side of the plane can see an activate screen at any one time, and will need to tell the other player which to press (front, mid, tail). 
• The screens need pressing at the same time to spawn the bosses and then disable the future release of gass. 
• Gass releases constantly on both sides, and above 15 stacks on your character, can cause serious movement issues. Failing to disable the gas will see you fail, as you'll simply be too confused to deal with enemies and the control panels.
• Repeatedly activating the panels will eventually raise the plane so that the arena becomes one location. You can then finish the new harder boss that spawns. 
• Be aware that there are two gas types: orange and purple. Purple makes enemies immune, orange allows you to deal damage to them. 

Tactics 

• Split your team into two teams of 4. 
• Assign one player on each side the responsibility of activating the visible screen. 
• Remember to use the term front, mid or tail to let your team-mate know which screen to press.
• Use the control screens before 15 stacks of poison (10 is ideal). 
• Have the screen users avoid firing at the boss, and instead kill the snipers who spawn on the gantry above (you won't gain aggro then). 
• Kill both bosses at the same time, to ensure that when the plane goes up, you've less to deal with.
• When the plane goes up, have everyone move to side B as a team.
• During the purple gass, just hide and heal. As soon as it turns orange, DPS the boss. 
• It's a good idea to clear the snipers on side B when the purple gas is up as they aren't immune from it. 

Boss 3: Lucy & Buddy

Probably the most annoying boss in a pug setting, Lucy and Buddy are a pair of twin robot dogs. The play space is small, and each needs killing at the same time. 

• Lucy is the squishier of the two robots, and suffers damage quickly. However, she has a "spin" attack that covers 360 degrees. When she uses it, it will one-shot any player who isn't behind cover.
• Buddy is the tougher of the two robots, as he can self heal. Before Lucy uses her spin attack, Buddy will release cluster mines which explode next to players, causing them to stand up (and then be killed by Lucy's attack). 
• You need to reduce the health of each robot at the same time, within the same parameters, in under 6 minutes. If you damage one robot too quickly, both robots will overcharge, and heal to full unless brought back under control. 
• A "Massive Overcharge" warning will occur if you don't keep the robots at the same health, resulting in 2 players having to activate the central laptops to cool them down. 

Tactics

• Split your team into 5 and 3. 
• Have 3 players tank Lucy in the top left corner of the play space, and Buddy in the bottom right corner (inside the Cera blue library). 
• DPS both - communicating all the time - and keeping an eye that their health bars are parallel. 
• When Buddy begins to squat, he's preparing to release seekers. They'll all land at the same time, so quickly shoot one to blow them all up (preventing your team suffering damage and knock-backs). 
• When Buddy has released his seekers, Lucy will fire her spin attack. Have all your team hide behind hard barriers so they can't be downed. 
• When Lucy has finished her spin, Buddy will begin to attack - you need to begin bursting him immediately as he'll rapidly regain health and if you don't lower it quick enough, he'll overcharge. Begin firing as soon as his seekers are killed, and Lucy's attack has stopped. 
• Use the laptops if each is overcharged, or if you can't bring their health under control. 
• Pack plenty of healing abilities so you can tank both Lucy and Buddy without having to move too much. 
• When both get low health, just go full ham with DPS: you'll down both even if they overcharge. 

Boss 5: DDP-52 Razorback

I will tell you now that Razorback in a pug setting is an absolute fucking nightmare. Excuse my language, but there's no other way to put it. It's a busy encounter, requires lots of coordination, survivability and damage. It also has a variety of ways to wipe you, and if one player isn't pulling their weight, you can faily hard and quick. 

• The central tank in the middle is the immobile "boss" and needs to be killed. 
• You can only damage its components by charging the 4 generators and then destroying its exposed components. 
• When you have charged the generators to 100%, you will need to damage the internals of the tank by destroying the front and rear panel, before throwing a grenade inside.
• Once you've done this, Drones will spawn and you're then free to move outside the generator radius (if you move out of them before, the phase will fail and a missile will be launched at you).
• You'll need to destroy the panels that appear on the raised sections of the tanks body, with two on the front (left and right) of the tank, and two on the rear. You have to do this after undertaking the above points across multiple phases. 
• After a panel has been destroyed, or after a period of time, the Razorback will launch missiles at all players (2 barrages). You need to dodge, or hide behind the computer cargo crate to avoid their fire. 
• If you destroyed a panel before the barrage, DPS the heart of the Razorback that opens up (right at the top). 
• Finally, after the missiles have launched a Heavy Black Tusk enemy will appear at Missile Silo 1, 2, 3, or 4. If he activates the button, you will all die. Kill him before he does. 
• You will need to repeat the above process to get the Razorback health to zero. Once this happens, your team will need to split again to the missile silos (that the Heavy Black Tusk walks to) and kill the rear control panel. Failure to kill all 4 will wipe the team. 
• Repeat the process (charge generators, shoot both weak points, throw grenades into the two holes, shoot the panel weak points, shoot the final weak point)

Tactics

• Have 4 Merciless rifles split across 8 players (it makes this encounter so much easier). If you don't have 4, have at least two Merciless players situated on generator 1 and 3. 
• Split your team into 4 teams of 2. 
• Send each team to generator 1, 2, 3 and 4 (with an even split of Merciless and DPS). 
• Have 1 player at each site solely focused on clearing enemies. Have the 2nd player at each site step inside it, at the same time as all other sites (have someone call this). 
• When all players step inside the generator fields, they should remain there until it hits 100%. DO NOT step out. 
• When it hits 100%, have 1 player at the front and the back of the Razerback hit the exposed panel with a Survivalist Crossbow Bolt (it kills the panel instantly). Have another player throw a grenade inside the exposed hole. 
• Have your players quickly clear the first wave of drones that will appear out the Razorback.
• Once the grenades have detonated, you can then leave the circles, and have ALL your team move to point one (left-front side of the Razorback). 
• Clear the second wave of drones quickly and deal as much damage as possible to the exposed component (above the Razorback on the left side). 
• After a period of time, or if you kill the exposed part, be prepared to dodge TWICE to avoid the incoming missile fire. You can also hide behind the console unit in front of the Razorback.
• When the missile barrage is complete, go back to your positions and wait for a Heavy Black Tusk soldier to venture to one of the missile silos. If he presses the button, you'll all wipe. Call it out, and have your Merciless players instantly kill him.
• After you've killed the heavy, quickly go back into the circles (you have about 90 seconds until another heavy spawns). 
• When an exposed panel is destroyed, the main computer of the Razorback will be vulnerable. Shooting this deals damage to the boss.
• Repeat the process until the Razorback's health reaches zero. Once it does, go back to your missile silo (each corner where the Heavy Black Tusk appears) and destroy the computer screen. Failing to do this will wipe you all. 

Good luck!