First of All There is no Easy way to do this, you need to be great at the game and have a coop partner that is great. Start up "Down On The Farm" Save All Your money Only buying Life and Decoys when needed, once you get to WAVE 23 Make sure you have 2 - 4 decoys it just helps, also a thumper or 2, even A Chain Gun Is Nice. once Thats Out, Or you just decide you can't take it anymore Jump into the top left corner and one player shoot down and the other shoot down-right and down when needed the zombies shouldn't even be able to get you. This Method doesn't work for the later Zombie waves until the boss is dead. I've Personally Made it to Wave 30 which is a Zombie Wave With a Boss Be Warned............ Only To Be Run Down Into The Grinder........... Good Luck.
Yes, 27 & 30 Are The Same, 28 and 29 ARE EASY <<< Just got the Achievement. Once you get to 27 Use about half your money on, Boar on both floors, you should already have decoy or 2 make a 3rd or even a 4th, A minigun, and maybe a turret, You must dunk the boss fast as you can then sit in the corner and use the method "LEFT CORNER SHOTGUNS" it will even stop the bombers from getting the left most bomb set. NOTE: Watch out for BIG SPEAR zombies (only the Spear one) and The WOMEN zombies They can Still get you just not that easy.
Last Wave My Co-op Partner Had 10,000 Cash He Made, Fire Turret, Boar, A Minigun turret, A Minigun, A Thumper, Once The Boss Was Dead He Used a Barriage, Then "LEFT CORNER SHOTGUNS"
1 Last Thing, Leave 1 Enemy From The Wave Before So You Can Have Enough Time To Spend You Cash<
GOOD LUCK!.....