As I expected, the bosses( yes, there are two coming one by one, one is a normal boss, the other is one in survival-mode-boss style) appeared, but were weak. After that, some common zombies came out but the screen soon went dark and then it turned into campaign mode's store...without high score record( excuse me? I have just got about 900000). It seems that you have that magma minigun from the beginning of the campaign mode, but when you start the game, you don't...Then I quit the game.

Unfortunately, it is LUCK that matters. You have to be lucky enough that you can just find a shield and then kill most of the zombies that surround you, especially the strongest tall black skeletons and those red missile launcher guys. You can be a stationed turret when you're invulnerable. After the most dangerous period passes, it is easy then to stay alive.

Although luck is important, there's still a trick-you can move around the boxes located at the bottom of the map.
When deadly missiles come from the red guys and those skeletons, move and let those missiles hit those blocks, meanwhile you should clear the area around and focus on the health packs and armors. You can get fewer hit and let armor more effective by doing so.

This guide was made on April.8, 2019. If the game is updated then, the ending might be different.