For this Achievement you have to Survive an entire round within the hedge maze on round 20 or higher. The Maze is found behind the lare mansion that houses the Mistress.

This is the easiest way to achieve it.

On Round 1. Find the Key to let out the big guy. He is located on the bottom in the house, on the left side, next to jug (Facing it). Don't kill the Zombies if not necessary. Try to stay on Round 1 as long as possible. Don't worry about getting hit, the Zombies are slow and if they should hit you, the chances of getting downed are low.

1 of you should grab the booze, you can find them only on 2 spots around the map, which are the salon or his cage. If you can't find them you can buy it for 1000 points in the salon on the bar, next to the piano.

The further away he is from the smashable stuff the more points you'll get for destroying it.
(Thanks for TFKitch for putting that into the comments)

As soon as you grab the Booze the big guy will follow you around, to make him run faster knife him but don't shoot him! If you shoot him, he will get scared and run back in his cage. If that should happen you have to repeat the step above and let him out again.

Face the big guy with his back towards the Juggernog perk, give him the booze and he should run and smash the stuff in front of it. You should get some points for it. Now repeat this step with the Church, the place next to the Candyshop, the Gunsmith, the cornor next to the gunsmith, the ghost house and the fountain.

Now its time to turn the Power on, you can go to where the Powerswitch is, without paying any money. Go up the stairs, next to where the box is. Jump across to the next building and follow it out into the barn. Downstairs in the burn is a little tunnel that you can follow (carefull that you dont fall down) which will lead you directly into the salon. Go to the other side of the salon and jump across to the next building now enter it and press the switch to the left. (This is also how you can get the candy without spending any money.)

If you want to stuck the box, simply go to the candy shop and grab the candy inside there, let the big guy follow you again and face him with his back towards the box, (make sure that there are no zombies near him!) he will than smash it and with that you can infinitly use the box. It wont move.

Now is the time to farm points for the perks you want! Don't worry about getting 4 perks. The ones that work the best are Jugg, Speed Cola, Double Tap and The Vultureaid Elixer. Jugg is located in the beginning near the box, between the 2 houses. Speed Cola is located in the Office next to the hanging machiene, the opposite side of the candy shop. Double Tap is located in the Ghoste House on the right side if you enter it. (You can see it from the outside). Thje Vultureaid Elixer is located in the Church on the back.

Grab Double Tap on the end. One of you should have more than 2000 points to open the Ghost house. After you entered go left instead of upstairs and spam the botton in front of the machiene.

Before you enter the Ghost House leave 2 Zombies alive.

Enter the Ghost House, the Wonder Weapon of the map buried can't kill the Ghostladies, it only slows them Down. So make sure you have a different weapon to kill them. Don't worry if they kiss you and suck your points out, you can get them easily back. Run through the house without standing still, as long as you are in the house they will spawn infinilty. If one of you goes down, don't try and risk it without quick revive, the chances of you getting down as well is very high.

After you've dropped down, face the bit where you dropped down. The Ghosts will still come after you, kill them, be carefull sometimes they come from the right side as well. After you've killed them they drop another perk. Don't worry if your dead friend hasn't got it, he can get one later. Now, find your way through the maze to the middle and shot some grenades into it. After it exploded you will see a watery flushing storm inside of it. This will bring you back later. Now go and find your way through the pack-a-punch machiene.

Farm some points to upgrade your Weapon. You will need 5000 Points to upgrade it. The Zombies will come down the stairs and on the left side out of the window, if you keep pressing F (default button) on the Window it should rebuild itself and give you some time.

After both of you have pack-a-punched one weapon, its time to go through the water flushing whole in the middle, make sure to leave once again 2 zombies alive.

After you jumped in it you'll find yourself at the starting area on the bottom, just jump down the slope again to go back to the bottom.

This Step is the key to this achievement. Both of you go to where Jug is and hold out there and farm the zombies until you are at round 20.

If you are at Round 20 leave yet again 2 Zombies alive and go through the Ghost House, go to the middle of the Maze and stand against the big bolder. The Zombies will come mostly from the opening in the maze into the middle so you can face them. Sometimes they will spawn next to you.

I hope this guide did help you, if it did, please leave a thumbs up and a star!