Tactical challenges:
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Successfully complete the indicated task to unlock the corresponding
challenge:

Challenge 1: Successfully complete Act 1 on the Normal or higher difficulty.
Challenge 2: Successfully complete Act 2 on the Normal or higher difficulty.
Challenge 3: Successfully complete Act 3 on the Normal or higher difficulty.
Challenge 4: Successfully complete Act 4 on the Normal or higher difficulty.
Challenge 5: Successfully complete Act 5 on the Normal or higher difficulty.
Challenge 6: Successfully complete all five challenges.

God Hard mode:
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At the title screen, rotate the Right Analog-stick clockwise or
counterclockwise twenty times to unlock the God Hard difficulty. If you
entered the code correctly, you will hear a sound. Alternately,
successfully complete the game on any difficulty.

Avoid losing weapon upgrades:
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When you die, you will lose a portion of your weapon upgrades. To
prevent this from happening, simply choose to return to the „Title
Screen” instead of retrying after you die. Then, choose to „Continue”
your game from the title screen to keep all your weapon upgrades.