Become a mercenary.

Complete Left Behind

Use bricks or bottles to lure an infected into attacking a human

Win the water gun fight

Defeat Black Fang without getting hit

Engage in all optional conversations

Build a home of at least §350,000 property value on a large lot.

Collect all comics

Break into every locked door using shivs

Find all training manuals

Complete A Gilded Cage.

Reach Sapienza Mastery Level 20.