1 - 2 Minutes
– Place your first Gun Tower two squares south of the top Laser Tower. Now, in a diagonal direction downwards, place another three so it leaves a gap leading straight into the southern Laser Tower.
– Mimic this tower arrangement at the entrance to the map, except once you reach as high as you can go, expand your maze to the right by a further three Gun Towers.
– This’ll force the units to run into both sets of lasers (and each Gun Tower will be chipping away at their health).
2 - 3 Minutes
– Once you’ve chalked up enough cash, buy a Spark Tower and place it as seen in the screenshot above. Upgrade it and then when you have sufficient money, buy a second Spark Tower and place it north-east of the last Spark Tower.
– This is more than enough to complete the challenge, but for added security, feel free to upgrade the guns closest to the unit’s spawn point. You should clear this with a few seconds to spare.