The level must either have a shop, or you need to get REALLY lucky with slot machines.

You must have unlocked the katana and grenade launcher, and have 20 Snake Coins (cheaper with perk).

If you haven't unlocked the katana and grenade launcher yet, scroll to the bottom of this guide for another exploit.

Start the level and purchase the katana, keep your preferred starting weapon and sell everything else on the table. The goal is to get the grenade launcher as fast as possible

As you hit things with the katana, buffs will drop (insta kill, double damage, unlimited ammo, invincibility, etc), touch them to pick them up, the katana should easily carry you through the first wave of any mission.

If you see a HIT ME coin, prioritize hitting it to get massive amounts of coins, one HIT ME coin is enough money for the grenade launcher.
You can walk up to HIT ME coin, and rapidly shake your hand in while holding a melee weapon or a gun to charge it up extremely quickly.

Once you have enough coins for the grenade launcher, buy it immediately or at the end of the round.

Now here's where the fun starts.

The katana will continue to give you buffs even when hitting enemy corpses.
Simply kill an enemy, walk up to their corpse, and rapidly shake your katana until you receive the INFINITE AMMO buff.

Some of you may already know where this is going.
Once you get the INFINITE AMMO buff, start rapidly spamming the grenade launcher at anything that moves until the buff wears out, this will destroy everything even on the hardest difficulties.
You don't need any ammo in the grenade launcher to start spamming as long as you have the buff, you don't even have to reload!

Only one buff may be active at a time, if you get another buff, simply chop up more corpses until you get your INFINITE AMMO back.

This trick also work on bosses if you save some regular enemies to hit with your katana.
Alternatively, swap to a higher single target damage weapon to destroy the boss.

Once the boss is dead, sell your katana to get the majority of your Snake Coins back.

Max out the perks that reduce shop buying price and increase selling price to minimize the usage of Snake Coins for the katana.

Level up perks that increase health and number of revives, at lower levels, it's easy to get one shotted by explosive zombies on high difficulties.

Level up perks that boost a specific weapon of your choice to help against bosses.
Shotgun is a great choice since the faster shotguns can melt bosses.
There is no need to level up melee or grenade launcher damage as they do more than enough damage already with this exploit.

If using locomotion instead of teleportation, you can increase your locomotion speed in the menu to walk a lot faster, making bosses trivial.

Once you cheese a couple of high difficulty missions, you should have enough levels and perk points to beat the rest of the missions without this exploit.

If you haven't unlocked the katana and grenade launcher yet, I have another exploit for you until you unlock them.

The first two missions have safe spots where the majority of enemies cannot hit you.

In the first mission, go to the second floor and walk through the railings in the back, prioritize killing explosive zombies as they can still hit you.

In the second mission, go under the stairs or on the outside of the railing on the stairs, explosive zombies and long arm zombies can still hit you.
If you go up high enough on the stairs outside the railing, enemy AIs will pause, allowing you to snipe them out, walk a little lower to let more enemies approach, and back up again to freeze them.

The blue circle under you is your actual hitbox, if your play area is big enough, you can leave your hitbox in a safe area, then walk away to attack enemies or pick up loot.

There's also a Snake Coin duplication glitch where you sell weapons for more than you buy them, but I have not found out exactly how it works yet, all I know is it has to do with co-op.