Resizing Windows 10 Bootcamp partition manually

Ok, here's how I solved my problem in the end (resizing a windows 10 bootcamp partition):

Shrinking the macOS partition

1. Boot into recovery mode and open terminal
2. Run diskutil cs list to list core storage logical volumes, and copy the Logical Volume UUID
3. Resize the logical volume with diskutil cs resizeStack LVUUID size where size is a parameter such as 80g (80 gb)

Expanding the Windows partition

1. Create an ubuntu live USB using this tutorial
2. Boot into the ubuntu live USB (hold option on boot up to select)
3. Open GParted and use it to expand the windows partition into the free space created earlier

Fixing the MBR so Windows boots

This method seemed to work, I don't know if it is correct practice but Windows does boot now.

1. Boot into mac OS and install gdisk (download the .pkg file and install)
2. Open terminal and run diskutil list, make note of the startup disk identifier (/dev/disk0, /dev/disk1, etc)
3. Run sudo gdisk disk_id replacing disk_id with the identifier from the previous step
4. Type x to enter expert mode
5. Type n to create a new protective MBR
6. Type w to save the changes and confirm with y
7. Reboot into Windows!

A final note of caution

I had backups of all of my important data in case something went terribly wrong and I was forced to re-install windows, macOS or both. Do not attempt this without backups of your data!

Be warned that I am no expert and put together this procedure with information from existing discussion board posts, so what worked for me may not work for you.