Kill It With Fire

Garden of Evil is where you need to be. Go and use molotov cocktails and your hairspray/lighter and just burn everything in sight that is A: a wall, amd B: green.

Good tip I found, so to conserve your hairspray bottles, is just lightly tap the trigger to quickly spray fire at the hedge. It will soon light up and save you wasting it unnecessarily.

You have to get almost all of it burnt. So take your time and check everywhere. Its not hard though, as the level is quite small.

You can easily get this Achievement on the Garden of Evil level by using the Trimmer/Weed Eater to cut down all of the Hedge Maze. Conserve your ammo/gas and you should have enough to destroy the entire hedge maze if not you can use fire to burn the rest.
I show this Achievement in my Walkthrough starting at 41:30 minutes into the video.

Another quick guide here. This needs to be completed on garden of evil. I recommend leaving this till after you’ve found all the upgrades and weapons. Once you have all the upgrades equip the cargo pants (you might need to refund the points and spend them again if you don’t have enough spare). The cargo pants give you unlimited ammo on most weapons. Also make sure you have found the flamethrower.

Next start garden of evil and just walk all around the edge flaming the hedges with the flamethrower.