Both Curses and Blessings can be bought with souls.
Another thing I really disliked is that, once you change factions, you'll have to buy the spells. In your first faction, you earn two of them, but after you swap they must be bought and can cost upwards of 15,000 and go as high as 125,000 souls. Crazy.
The achievements in the game seem to be fairly simple, some will require soul grinding (or buying) as you'll need to buy Curses or Blessings, or fully upgrade a spell. Nothing seems to be incredibly difficult.
And in a first that I've seen, beta testers can unlock achievements during beta, and they'll count towards your score. Although this makes it so some will have to buy it, as some features aren't yet available (the endless dungeon and co-op, for example).
*updates to come.