This is arguably the hardest level in the entire game. After beating it I looked on the internet to see other player's methods and found them to be a little hectic. My layout is simple, requires no items, no split second timings and you only need to sell one tower.

Talking of towers, you'll need Gatling, Glue, Gas, Tesla and Pyro.

If you need to pause the game in order to check these notes, you can't use pause because the timer keeps going but you can hit Esc and then continue.

You'll have a few seconds before the timer starts. Put down the first Gatling, upgrade it twice to level 3 and then hit fast forward (Hot Key F). It's best to place towers as soon as you have the cash but the timing never needs to be absolutely precise. You must place the towers in the order indicated by the screenshot.

The only other Gatling you need to upgrade is No.7. Upgrade to L2 and L3 when you can (do L3 quite fast).

After 47 kills you'll reach the end of a wave with $35, sell Gatling No.1 and replace it with a Gas Tower.

This is what your screen will look like once you've placed the Gas Tower. The screenshot shows a L1 Gas Tower, upgrade it to L2 and L3 when you can.

You now have a L3 Gas Tower. Place and upgrade the following in this order:

L1 Glue
L1 Pyro
Upgrade Glue to L2 then L3
Upgrade Pyro to L2 then L3

Now wait until the 1st tank arrives, once it does look out for the "Red Arrow" indicating a 2nd tank is about to arrive (see screenshot). Don't wait for the 2nd tank to appear, when you see the red arrow hit the Airstrike (Hot Key R).

Once you've done this you'll have enough cash to place a Tesla above the Gas Tower.

You've now got a Tesla above the Gas Tower. Upgrade the Tesla to L2 and L3 when you can and that's it.

This is the easiest method to explain, I can do it quicker but it only complicates the instructions.

Also I've only tested this on my one PC. I don't know if different computers with different FPS will prevent this method from working.

If you do have any problems with it, let me know and depending on where it's failing I can tell you which of the Gatling Towers to upgrade further or you can actually use a L3 Machine Gun instead of Gatling No.7, you just need to place the Machine Gun the exact moment you reach $8. You'll also knock a second off the time.