I cleared most of the game and still have found no way to reliably generate focus. The tools in game for that seem to be bugged? For me and more players things like Focus Regeneration seems to not be working. This basically makes mage builds non existent as you have to melee constantly to generate focus OR you have to chug potions like an addict.
Runes are very powerful though and I see mage builds blasting like mad when they have full focus all the time. It needs to be balanced out in the end because of that. However, the way it is now, (pure) mage builds are basically non existent.