The following instructions are heavily inspired by /u/uhohohdear. I modified uhohohdear's instructions and then added support for Retina/HiDPI displays and instructions about how to update MTGA.
IMPORTANT UPDATE: This will no longer work if you upgrade to macOS Catalina, as Wine and Wineskin are 32-bit. I'll update this Gist with 64-bit versions when they become available.
We're back in business!
It appears that the workarounds no longer work. A few people with Macs using Nvidia graphics cards are having success, but Macs with Intel and AMD GPUs crash regularly. At this time, the best way to play Arena on Mac is via Boot Camp or GeForce Now.