Not all of Nier Replicant's sidequests are mandatory, but if you want to collect all the weapons and unlock any endings beyond Ending B, you'll need to complete A Bridge in Peril. This one isn't nearly as long as quests like The Damaged Map, so it should make for a quick clear, and easy rewards.
First, you'll need to make sure you've acquired the fragment from the Lost Shrine. It's advised you go ahead and unlock this quest as soon as you can, because the Shade you need to slay doesn't always appear immediately upon accepting the quest.
Updated on April 3, 2023 by Quinton O'Connor: We tidied things up a tad to ensure this quick and breezy guide is as easy to read as the quest is to complete.
How To Complete A Bridge In Peril
To kick things off, head to the Northern Gate and locate a guard meandering about. He'll tell you that a monstrous Shade killed his wife and injured his child. Agree to find the Shade, which is a bit easier said than done.
Head to the Northern Plains and locate the wooden bridge stretching across the canyon, it's easiest to get there using one of the giant boars lurking about. Once you approach the bridge, the giant Shade should pop up - but don't worry if it doesn't. Your enemy should pop up somewhere around in this area:
Triggering the Shade's appearance isn't a guarantee every time you wander into the zone, so you may need to try checking back in on the area a few times before you see the big bad and its minions pop up.
The quest log's hint mentions the creature prefers cloudy weather, so when entering the Northern Plains, look up at the sky. If it looks a little cloudy, head down the bridge and see if the Shade is there.
If you're having trouble getting the Shade to pop up, one older method from the original version of Nier Replicant/Gestalt involved saving in your village, exiting to the Northern Plans, and running up to the bridge to see if the Shade is there.
If not, simply restart the game, reload the save from your village, and run out to the Northern Plains again. This method should be faster than running in and out of the zone, since it's so large.
The Shade spawns with a ton of smaller enemies and hurls a spray of orbs at you. This creature will be one of the stronger enemies you've encountered so far, so make sure to adjust your Words and equip a stronger item, and be careful not to let the Shade knock you off of the cliff into the canyon below.
Reward
Once you've killed the Shade, head back to the guard at the gate. He'll give you the 30,000 gold reward and Labyrinth's Whisper.