Press F2 to toggle Fullscreen/Windowed.
Press F1 to mute/unmute the game.

The method explained below will get you all the achievements in ~1 hour or less.
If you also backup your save after grinding a bit, you can cut that time to ~20 minutes.

Start a 'Co-op Survival' game, join with first player (CTRL key) and press ENTER To start.
Pick any name (by typing it), any perk and bonus, choose the first level and then start the wave by walking over the "start wave" icon.
Press ESC, then "View Results" to earn a coin.
Press on "try again" and repeat the last step to earn another coin. Keep redoing that until having enough to buy everything.

Note: It's highly recommended to buy challenge mode as soon as you have 20 coins, as this mode awards with 3 coins instead of just one, speeding up the process by a lot. Go to 'Extras mode > Challenge' to buy it.
Credits to marklar

You will need a total of 470 coins to purchase all the goodies.
However, you can backup your save at any time (with 50-100 coins recommended) and purchase everything you can, load your backup, purchase again and repeat until having everything.
Note: Reloading your backup save won't reset your purchases.

Save is located at:
AppData\Roaming\MMFApplications\zombiecoin.ini


Modes: 85

• Classic: 5
  
• Versus: 20
  
• Human: 25
  
• Snowman: 35 (this is mislabeled as 30 in the game)
  
• Challenge mode doesn't have any achievements attached and can be skipped.

Maps: 175

• Lab Land: 15
  
• Ice Land: 20
  
• Desert Land: 20
  
• Swamp Land: 25
  
• Beach Land: 25
  
• Graveyard Land: 30
  
• Volcano Land: 40

Options = 210

• Hardcore Mode: 0
  
• Meat Mode: 20
  
• Tougher Enemies: 10
  
• Shorter Waves: 10
  
• Increasing Zombie Speed: 5
  
• Safe Upgrade Area: 5
  
• Boss Wave Pausing: 5
  
• Pause Between Waves: 10
  
• Guns in Upgrade Shack: 20
  
• Infinite Ammo: 30
  
• Infinite Health: 30
  
• 999 Starting Cash: 35
  
• Starting Wave: 30

To unlock Endless Mode, you'll need to beat wave 50 on any map.
This can be easily done if you pick the following upgrades:

• Shorter Waves
  
• Infinite Ammo
  
• Infinite Health
  
• 999 Starting Cash

Also set the starting wave at 50 and choose any level.

As soon as the game starts, head into the 'Upgrade Shack' and buy the second upgrade (ctrl key) to increase your damage.


Spend all your cash on that.
Then start the wave by walking over the 'start wave' icon and win by killing all the enemies/bosses.
Hint: Can pretty much stand still while holding CTRL key to win.
Once you've won, click on 'Endless Mode' to unlock the achievement.