1) Build up the right sandbags maze as fast as you can.
Return to the same place every next 3 nights to build more sandbag maze. For example, choose 'Warehouse' at 1st,4th,7th,10th... night.
Let zombies go straight to you in single lines as much as they can BEFORE they spread out to eat someone brain.
One of the most powerful maze is "L" shape maze in the early night. It lets zombies run straight up to you in a single line then eventually turn straight right or left in front of your heroes using the sandbags wall.
Be aware that the zombies can move diagonal. Straight line sandbags wall without allowing zombies to turn right or left is the most effective.
Create some 'killing path' where there are few sandbags blocking your fire sight and are also choke point to gather zombies together. My perfect maze will have 3 'killing path'. First, first path where zombies appear and go straight to you. Second, where zombies were mocked to go around then eventually have to run back pass the Four, right in front of our heroes. That is lethal because close range also mean high accuracy. Lastly, the third, where is the final corner after zombies go pass all the maze. It is the final ground in front of the heroes before they can reach them to eat their brains.
2) Use the right guns, not seemingly strong guns.
Equip the gun according to the guide section. You will be surprised to see that you can complete 20th night with just 4 Hunting Rifles.
Use AK-74, Hunting Rifle, Magnum.
3) Use focus aim to cover zombie's attack route.
If you build up the great maze, you need 2 heroes to aim to the final corner, you need 1 to aim right at your 4 heroes to protect them against the leapers.
4) Use theright special item at the right time.
When using Molotov Cocktail, be aware that it need sometime before it can kills zombies. Zombies' HP will gradually drop to zero but NOT instantly. It MAY NOT helps you at all if you throw it carelessly.
Molotov Cocktails are more effective if you have burst-shot type weapon. But it may not helps you much if you use only Hunting Rifle except that you are fighting against Brute.
Detonates the Bomb when the red Brute is in the bomb's range with a lot of other zombies. It is a guarantee instant-kill.
Bomb WILL kills your heroes too if they are in range :P
5) Balance search time spending during each day.
Early days when you are lacked of weapons, you should spend much search time enough to guarantees getting 4 items.
When you get enough Hunting Rifles and Magnums, you need not much the time to spend for search.