alrighty
you start off by building 1 and 3. Max out the damage on each. Then upgrade the range on blue's tower just enough so it reaches the spawn point, and guarantees a combo shot from the blue tower every level even when the creeps are dieing instantly
You are now shooting out cluster bombs.
Be sure
not to upgrade any tower's fire rate, and also don't upgrade any tower's range (besides the number 1 tower)
Next build 4 and max out it's damage. You are now shooting laser cluster bombs.
Next build 5 and max out it's damage. You are now shooting big laser cluster bombs.
Next build 2 and max out it's damage. You are now laying big laser cluster bomb landmines.
Now, build some laser towers like 7. This is very cheap, only 15 per tower, and let's your laser tower reach near the beginning by relaying through these towers. I wasn't aware of it at the time, but you can build the first one right next to tower number 3 and tower number 6 (while I was playing I thought I might have needed that space for an 8.5k tower)
Follow a path similar to mine so you can shoot near the spawn point.
However, you want the range of your lasers to be just outside the spawn point. If the range includes the spawn point, at the beginning of each level your tower number 4 will fire at the same time (by way of the chain) as your tower number 1 will fire, and it fuks up your 5 tower combo (you'll shoot out some poison rockets and other useless crap)
Now we wait until the 8.5k tower becomes available. Some one in this thread says it doubles the damage. This tower will become available on level 100
As soon as it's available, as quick as you can build 2 (cause that's all you have the money for) of these. You want to pack them in nice and snug like I did. It only has to cover the center pixel of the number 1 blue tower to be effective (since that's where the combo shots are fired from)
If you do it right, all the 8.5k towers will reach the number 1 tower without you having to upgrade the range
As soon as the 3.5k tower is available, build one of them. It becomes available on level 110. Make sure it covers the centre of tower number 5 and tower number 6, as well as tower number 1. If you don't, tower number 5 or 6 might out-range tower number 1, screwing up your combo shots.
I am guessing the 3.5k tower does the same thing as the 8.5k tower but reduces range and fire rate. Well, fire rate is near useless in the end game (combos can fire every 8 seconds, this cannot be changed), and the number 1 tower can still reach the spawn point even with the reduced range. But you may have to upgrade the range on the number 1 tower, probably to maximum, to make sure it reaches the spawn point and fires a combo every level.
Unfortunately, when checking the range of towers, it does not take into effect the negative modifier of the 3.5k tower, so you just have to guess and check if your number 1 blue tower range is sufficient
After this, you simply build an 8.5k tower whenever you have 8.5k+
When it looks like you are about to die, spend all your money. The way I do it is click a tower, max out it's fire rate, then spam some towers. the spamming can be done with hot keys. Typing 1 will select a blue tower, 2 selects a green, and 3 selects a red. So building a tower is one key press with your left hand, and one mouse click with your right hand.
I have taken this game way too seriously.