Here's what I did to fix my Nexus. Note that there are different types of bootloops. My 5X's bootloop went something like this: Phone powers on, Google logo shows briefly, phone powers off, repeat. Disclaimer: I am not responsible for any further damage this may cause to your device, nor am I responsible for anything that may occur in the future that as a result of this guide. Please do not follow this guide if your issue is not exactly the same as mine.
Resources

    Nexus 5X System Images/Modified Boot .imgs

    Easy ADB/Fastboot installer

Prerequisites

    Bootloader unlocked Nexus 5X

    ADB/Fastboot Installed system-wide on your PC

    Nexus 5X system images/modded boot and recovery images

    Windows PC

1. Unlocking the bootloader

First, we're gonna verify that your bootloader is unlocked. Make sure your phone has some charge left in it. Power the phone off completely. Once off, press and hold the Power and Volume Down keys simultaneously. This should put the phone into Fastboot mode, which looks like this.

At the bottom, under Device State, it will say either "Locked" or "Unlocked." If it says unlocked, skip this section. If locked, proceed in this section.

My bootloop was caused by a fault in the high performance core cluster of the Snapdragon 808 SOC that's in the 5X. So in order to boot the phone, I took a hairdryer to it. But before you do this, you need to know what options to change when access is gained. When you gain access to the OS, open settings, scroll down to the bottom, and pick System. If you see developer options, pick it, find "OEM Unlocking," and enable it. Continue scrolling and enable "USB Debugging" as well. If you don't see developer options, press About, scroll down to the Build number, and tap repeatedly until you are a Developer. Then do the steps above. Okay, now get a hairdryer, set it to max heat and fan speed, power up the phone and blast that sucker right at the top half on the phone. This thermally overloads the two high performance cores, disabling them and temporarily allowing you access into the OS. After some time, the Google logo should stay displayed on the phone for much longer than normal. One this occurs, the phone should restart (if not, hold the power key until it does) and boot into Android. See the bold section above to enable OEM unlocking. Then shut the phone off.

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With that done, see resource 2 at the top of the page. Download it to your PC and run the executable. Answer "y" to all of the questions you are prompted with. Now hold down the power and volume down buttons on the phone to boot into fastboot and plug the phone into your PC. Press the Windows Key + R to open the Run dialog, and then type "cmd" (without quotes). With the CMD open, type, again without quotes, "fastboot devices" to make sure the USB drivers installed successfully. You should see a serial number pop up if done successfully. Now type "fastboot flashing unlock". Note that this will wipe your phone! If done successfully, your device status should now be Unlocked.
2. Flashing the images

Go to the first resource linked above, and scroll down to the second post with months and then an android release after it (i.e. June - 8.1). Download the latest monthly system image and the two images contained in the Mega link into an empty folder on your PC. Extract the system image folder (the zip called "bullhead-[version]) into the folder you created. Don't do anything with it yet. First we're gonna flash the modded recovery image.

Open a CMD window by typing "cmd" (no quotes) into the address bar of the Explorer window where you saved the recovery.img. This opens a CMD window that is already pointed at the directory we need. Then type "fastboot flash recovery recovery-fixed.img" and let the flash finish. Once it's done, leave the CMD window open and power the phone off and reboot it into fastboot like we did above. Now find the system image folder you extracted and open it. Inside, you should see "flash-all.bat". Run it, this may take a few minutes. This flashes a stock, completely unmodded version of Android onto the phone. When the flashing completes, the phone should restart but it will bootloop. This is normal. Hold the power key and volume down to shut the phone off and boot back into fastboot.

Finally, we're going to flash the bootloop fix. Go back to the original CMD window you flashed the recovery.img from. Type "fastboot flash boot boot-fixed.img" (again no quotes) and press enter. The boot image will flash. Power the phone off from fastboot using the volume keys to select Power off and the power key to select it. Then power it up again and it should boot to Android setup. Whew.