While I had the 2013 mac pro I was using a Lacie thunderbolt ssd drive to run windows 8.0 and later 8.1 so I could save the valuable internal ssd space for Mac OS. This method has nothing to do with bootcamp or virtualisation. After the procedure is completed, you boot windows natively just like a PC (bootcamp actually does the same, but it will work only with the internal drive). The whole point of this procedure is to trick windows to "think" that they are booting from an internal drive (because normally they do not support external drives as system disks).
Also worth mentioning; this method does not need a boot manager since Mac has already one. Holding down alt key while booting (and as long as your external windows ssd is connected) gives you the option to choose between booting windows or macOS (or, in other words, from the external or the internal disk, respectively).
After booting windows, you can install the bootcamp driver pack, but regarding GPU drivers you might be better with the ones from AMD (they are significantly faster and they get constantly updated, unlike the ones apple provides).
The external SSD can be connected via thunderbolt or USB 3.0 (with the former obviously being faster). Anything else will not work.
The instructions I followed can be found
here.