The first option is to pull out all the stops the law will allow. It is true that if you look at the small print of the current Spanish constitution, it is actually going to be very difficult. But that does not mean it is impossible.
In the end the decisions are fundamentally political, and not strictly legal.Now, if there is no political will [to allow independence], then there would have to be some sort of break with the current laws. Or perhaps the support of international institutions would have to be sought; or, we could force a change and deal with the technicalities later.
After all, in the 20th Century, the majority of countries seeking independence achieved it not through legal means but through the will of the people.