You should never get too low on your hard drive space as the OS needs a minimum 10 - 15 GB of empty space at all times to function properly. As well, if you do any movie or photo/image editing, rendering and editing works best if you have at least as much empty space as you have temporary files in your project; I can easily amass 100 GB or more of temp. files for a one hour HD movie, so I keep a lot of hard drive space empty.
So, in order to get some stuff off your hard drive, the usual space hoggers are movies, photos, etc. You can move all of those in their library to an external hard drive and, after making sure they work there, delete them from your hard drive.
Before you install macOS Sierra on the hard drive, you will need to clean the external device. Ensure that there is no important data on your hard drive. Then plug it into your Mac, go to Finder. Some installers do let you choose where to put the application. In those cases you can at least try to install on an external hard drive. As we've already mentioned one of the reasons that applications need to be in the applications folder is due to the expectations of the updater application. One of the great things about Mac OS applications is that they are packages. Application.app is a folder. You can right/CTRL click any app and select “show contents” to view all the program files.
Install Mac Apps External Hard Drives
In general, application work best if they are installed on your internal hard drive; I am not a gamer, so I am not familiar with the specifics of the game you are referring to. Googling it, I could not find any reference to Macs on their website - here it shows the platforms it is compatible with: