I've added all of my videos to this solution which are without mutators and on Extreme difficulty. Good luck and I hope my videos help you.
In short you have to complete every level in the game on insane difficulty whether you go solo (not recommended) with bots or online with other players. You can also play private matches with friends rather than public and it still counts however you cannot use MUTATORS.
This will void your tally for completing it but you will still get the full amount of coins dependant on the mutators you use, just not the completion of the level.
You dont have be alive at the end for it to count just as long as someone survives and finishes it.
The game will track which levels you've done with the red skulls under each levels picture.
I highly recommend playing through the story a couple of times so you get an idea of what the levels are about and where to go. Then focus on ranking up your classes to 30. The fixer, medic, exterminator and hellraiser are the best classes I think but everyone plays better in their own unique classes so give them all a try and find which one you prefer. Also any weapons you like using its worth grinding the kills to get the max upgrade for each weapon. The advanced smg, assault carbine and shotgun are by far my favourite weapons and cause carnage when upgraded fully.
Stick together, play stealthily and above all
ALWAYS USE SILENCERS.
In my Moscow 1 and 2 videos one of my pals was using an unsilenced weapon throughout and we got pretty much endless hordes, we thought it was a bug but it was only after we finished both levels he was like oops XD
The classes all have their own unique perks, the medic is handy for stims, healing properties and having masking on stim.
The exterminator is great for crowd control especially with the RPG, also you get a 20% chance of getting a duplicate defense kits which is a life saver on finalies.
The fixer can start with a breach and use masking grenades.
The hellraiser is great for claymores, can also start with a breach and a beasty MGL also their explosive perks are awesome.
Lurker/hider - listen out for these buggers they make a very distinctive noise kind of like a gurgling sound when they're nearby so check your corners when proceeding. All specials pretty much have random spawns but the lurker sometimes turns up in similar spots.
Infecter - easy to take care of just try to strafe to dodge her spit and aim for the head, as soon as she goes down on her knees go for the melee to instakill.
Bull - when he charges, sprint to the left or right or if there are any large obstacles try to run behind them, sometimes he runs straight around corners so keep that in mind. His weak spot is his back so once he's charged go for the melee or shoot either way.
Gasbag - easy to take care of just aim for the head and watch out for the radioactive cloud, if it blocks your way just wait for it to dissipate.
Bomber - not much to say for this one really just listen for the ticking sound and when you see him shoot. If he gets too close you can melee him to knock him back.
Also if your brave you can shoot him in the legs and melee him on the floor this will give you random loot. Risky though.
Hell and high water
Dead in the water
This is unbelievably easier on insane as you only actually need to turn on one electrical grid in the finalie and it's pretty much over. On extreme you have to do 3!!
Anywho I think this has to be one of the hardest levels in the game due to the finalie.
If you want, you can pretty much run through the first part to the first safe room but make good use of masking grenades.
The second part on the bridge you just have to watch out for claymores and make sure you pick off any z's before they spot you. If you get through sneakily and jump down you can pretty much run to the elevator too. In the car park you'll either get claymores or auto turrets so beware.
If you trigger the swarm on the bridge pop a masking and run. Once at the car park go into the room on your right when you reach the bottom of the ramp and close yourself in. As long as you dont let too many zombies push the door for too long it won't burst open so you can easily pick off the horde. Just beware infecter can spit at you.
Right the hardest part. Basically you have to defend 3 parts of the hospital - the front, left and right wards and potentially a 4th which is by the cliffs.
I recommend putting an auto turret in each ward one on the left and one on the right. If you manage to get a 3rd put it in front of the main hospital doors. When playing this level I normally have one person in the left ward by the auto turret and same in the right. The other two stay on the top left upstairs in the room with the hospital beds overlooking the outside. Preferably one of them with the flaregun.
If you get in trouble up there you can double back to the left or right ward with the autoturrets.
I recommend having subtitles on as once you've defended you get told over the radio that you need to restore power to the electrical grid in a certain area which can be one of 3 places, the morgue, upstairs in the left side of the hospital or downstairs also on the left side of the hospital.
It's best to save a masking grenade or stim shot so you and any comrades still alive can together make a break for the button as the swarm will just keep coming until the grid is turned back on.
After you pull the switch its either the end of the level and you have to go back to the the front of the hospital or you have to make a break for the cliff entrance and defend for the last time.
Luckily on insane it will show you where you have to go so you know which buttons to press etc.
Dead Sea stroll
A sign from above
Key to the city
Battle of nerves