What does the rulebook mean by 'natural' rolls of 1s and 2s? If I 6-die a spell and manage to meet its casting value with ONLY 1s and 2s (let's say, '1-1-1-1-2-2') does it go off, or does it fail?
They mean that you rolled a 1 or a 2 as your total. This is why rolling 1 dice is dangerous. If you roll a 1 or a 2 on 1 dice, you auto fail. If you roll a 1 and a 1 on 2 dice, you auto fail. If you roll a 1, a 1, and a 1 on 3 dice, you only fail if you did not meet the casting cost.