100 Zombies Level 21 Walkthrough:
1. Pick up the gun quickly and use it to shoot all the zombies that appear.
This is the order and location they will show up:
1st = left window
2nd = right window
3rd = left window
4th = left side of room
5th and 6th = left and right window
Tap your gun in your inventory to de-select it and press the door to open it.
If the door doesn't open after killing the 6 zombies, tap the door again and it will stop the timer.
2. Proceed to the next 100 Zombies level!
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