I suggest playing Earth Shaker since this is by far the most easiest to complete on Hard
These are the classes that should be played that follows
2 Riflemans using machine guns (You could throw in an engineer if you want but it should be only 1)
2 Snipers using semi auto or full auto rifles
1 Medic using semi auto shotguns
Stock up on gear like using Scout Ammo Pack, Portable Defibrillators & use Warlord & Crown gear for health replenish and if at some point the team is way overwhelmed use smoke grenades.
When the mission starts you & your team would be in the chopper there would be 2 Markmans shooting at you from their base, their not too much of a hassle they can be ignored but is best to kill them, and there would be guards at the bottom platform as well. Now move forward to the checkpoint but before go any close to the entrance way be very very mindful because there is multiple enemies and a Heavy Soldier that could wipe out the whole team at this section so be careful at this spot and stay back and kill them all and then proceed to elevator.
Now move out if you stay too long in the elevator, gas would emit and kill you
Take cover behind machines enemies would spawn up above from left and right, after you killed the human enemies and Turret will pop up know as "Cobra", everyone sprays bullets at it and it should be destroyed within seconds.
After that is destroyed 1 person needs to run past the cover that is right ahead of the elevator to trigger the next objective they should run back immediately, 3 cobra's will pop up this time, the Snipers should be focusing on destroying it while the rifleman's kill any excess enemies after that is completed move on to the elevator.
Now this wave can very difficult you can either have 1 person to go all the way to the other cover to kill enemies that would be spawning out or have everyone behind the same cover at the beginning to kill the Turret which is the "Maurena" the turret shoots in bursts so time your shoots wisely after that is destroyed someone needs to go ahead and trigger the next objective is running by the 3rd doors. Now right beside that door is where 2 Maurena's will eject so take behind the cover that is in front of Maurena's or all the way at the back.
Someone again needs to go and trigger the next objective which is by going to the double cover, a Cobra will spawn and this time it's moving so it won't be as easy to destroy, and enemies will also spawn along with them. After that is destroyed 2 Maurena's will spawn so destroy them and move on the next elevator.
Everyone takes cover and shoots at the Maurena that will be in centre so aim and get ready, after 2 more will pop up from left and right so destroy them quickly as well. Now to trigger the next objective just run past the green strip on the floor where you destroyed the Maurena's. Now Maurena's will spawn so just take cover and shoot. Now a new turret will come into play, the Hornet it endlessly move sidewards and can take you out with 1 hit (you could survive the hit on rare but just barely though) So time your shots, it's best that Snipers take them out because of their range & damage output.
Someone has to go and trigger the objective which is by running past between the covers and Maurena's will pop so shoot destroy and move on to the elevator.
Now before everyone exits the elevator, strategize first, since there is not enough cover for everybody you must destroy the turrets quickly as possible 3 Cobra's will spawn 2 on the right side and 1 on the left of the map, I would have the medic go prone and blast the Cobra on the left side and have a rifleman in the middle cover and 2 others on the right side. Have 1 person just wait out of elevator until they don't feel any gas so everyone can take their positions. Now move forward and trigger the objective. About 3 Cobra's will pop up so just destroy them and that should be it and go the elevator.
Human enemies will spawn above left and right, and a dangerous enemy the Demoman will come into play, they carry rocket launchers and do a good chunk of damage if hit by them so be careful and take them quickly or your whole team is wiped out. After you killed them 2 Cobra's will spawn left & right so just take cover and destroy them oh & watch out for Heavy Soldiers they spawn on this wave so watch out for them and kill on site. Now to trigger the next objective just run past the destroyed Cobra's.
Now 2 Cobra's will pop up and they are behind covers so just destroy them quickly and watch out for Demoman's and soldiers and move to the elevator.
Now you will meet new turrets! Armadillos, As Oberon mentioned the most effective way to destroy them is by throwing nades into hatch, throw them at a sidewards angle not when you literally right in front of because that's very very bad just don't do that okay? good use the covers as well and enemies will spawn and after each Armadillo you destroy a Maurena will pop up so (expect for the first section of Armadillos) if everyone on team runs out of nades either just shoot at the Armadillos or die and use a resurrection coin and use a the new nade after you done all that hard work move on.
My oh my where do I even begin this where most likely everyone will die... okay everyone take behind covers 2 on the left and 3 right or vice versa does not matter and kill the Cobra as quickly as possible, everyone must be shooting at it. After that is destroyed Armadillo will pop up from the left side until the end of elevator so again destroy them will nades.
Someone must go and trigger the objective a Cobra it will pop in and out so it won't hurt you and your teammates too much. After that Cobra is desroyed, have the Armadillo destroyed anyway you choose. Now have a Sniper all the way at the back of the map behind the grey cover before someone goes and triggers the next objective to trigger just run all the to the end of map which is by the elevator and the person should be backed up against the grey cover because a Heavy Soldier will spawn. Sniper kills the turret and happy days go the elevator.
Hornets are back at this wave at the beginning of this wave, 2 Snipers should rush to the covers between them to take them out quickly while the remaining 3 just behind the big cover and shoot at the Hornets also. Now before moving any further the Sniper should stay put because another Hornet will pop up, have someone to trigger the objective by running to the other covers. New enemies will spawn they carry Blasters and they do BIG damage & 2 Cobra's will spawn, have Rifleman's killing the Rocket Blasters and Snipers destroying the turrets and now move on to the elevator.
This wave is more chill and you can relax a little bit, it is only guards with batons, not much I can put on here than just avoid getting hit and do headshots.
Go and take behind covers quickly because Rocket Blasters here, shoot them in the head because it's much quicker than the limbs. Now after they're all dead stay behind covers because 2 Hornets will spawn and multiple guards with batons will endlessly spawn until you take out the 2 Hornet so everyone or the 2 Snipers should be shooting at the Hornets. After that is done a Maurena will pop up so just stay behind cover as much as possible shoot at it. Move forward to the next objective which is going to any of the cover's, & Maurena & Hornet will pop up, take out the Hornet first then the Maurena and go to the elevator.
When you exit out the 2 Cobras will spawn both left and right just have you and your team destroy them quickly. Now a new boss will appear The Mammoth similar to the Heavy Gunner you have to shoot at it the back multiple enemies do spawn so always watch your back. Try circling the Mammoth or have teammates at one end of the other to have it focus on a teammate while he/she is in cover and take opportunity.
When you have the mammoth destroyed 2 Maurena's will pop up at the bottom 2 sections, take behind covers at the opposite end right before the Maurena's to get good shot at them. Again another wild Mammoth appears! just do the same shit, try circling and get it at the back and beware of Armadillo's to. When you destroyed turrets move on to the elevator.
Alright we're almost there! Everyone must take behind cover, 2 Cobra turrets will popup moving sidewards, I suggest shooting in bursts crouching up and down. Now that they are destroyed, 1 or 2 person(s) goes to the bottom section, before triggering have a grenade ready in hand because 2 Armadillos pop up throw the nades into hatches. To trigger the next objective just run up the little stairs and about 3 Cobra's pop up, Snipers should be taking them out and Riflemans & Medic taking out enemies. After that is done move to next section of the map an elevator door will open, when it does Rocket Blasters will be waiting for you, do not rush at them guns blazing, throw grenades at them and shoot at them behind any cover you have and when they are all dead move to the elevator and pick up the Blasters you will need them.
When you arrive at the destination a turret will be shooting at you just destroy it quickly, when you exit the elevator shoot at the silver/blue like covers with the blasters because they get in the way so destroy all of them. Go to the Green marker Oberon speaks, hide behind the pillars when they rise up and shoot at the Cobra's across the get Ursula/Architect the Main Boss to able to be killed, when she opens up when the Cobra's are destroyed so shoot at her when given the opportunity, just keep repeating until you and your team destroyed Ursula.
And Congratulations Soldier!