Very simple mod, have seen a few requests for this to be an official thing, not sure if that’s ever gonna happen so here is a mod to do it instead for those who are desperate to play as a pure arcanist.
Installation is simple, open the 7z file, extract the folder into your …/grim dawn/mods/ folder and fire up the game. Change to ‘Custom Game’ instead of main campaign, and load the mod called “masteryspecialist”. Create a new character and go at it. If you already have a character you want to use, see here. There is an explanation on how to do it as well as a video link.