I got all the weapons at level 17 (except the last big one) and at level 19 all the zombie-soldiers started coming and made an endless row against the wall... I switched from AK47 to shotgun and they simply didn't die, so that was the first time I had to hit Retry.
The game did not seem to me as hard as people say in the comments, but it takes the right management to keep going and I understand how frustrating it can be when the barricade keeps crashing. Use automatic guns (uzi has lots of bullets in each cartridge), do the math to repair the wall (only spend the needed hours to repair it until 90%), spend most to survivors search in the beginning of the game and once you have a full house, spend all in weapons search and the game becomes pretty easy.
It's not a compelling game (it becomes boring after a few days), but you wanna keep going until you have'em all and check all kinds of zombies that show up. My personal favorite was the clown "thing".
***SPOILERS****
That last weapon is what, the all mighty piercer? I used it in the last day (yes, the 20th is the last... Why do you give us only one chance to use that weapon?) Basically the chainsaw saved the day, once the wall of zombie-soldiers came crashing to the barricade the big gun was utterly crap... Pointing the tip of the chainsaw to their heads seems to get them down faster. Btw, nice helicopters... Really?! What kind of end is that? No boss?! No hippopotamus-super-fat-zombie to blow up? What about explosives? Heavy machine guns?... You really dind't expect anyone to actually comment the end, right? But here I am saying... Nice job, but try harder ;)