I must confess that I I am somewhat surprised that anyone should wish to prune a superb magnolia. These magnificent plants should really only be planted where they can grow unfettered.
However, if needs absolutely must, they may safely be pruned immediately after flowering.
As to drastic pruning - this suggests the removal of comparatively large branches, which can be carried out at the same time, but is best spread out by not removing more than one or two each year.
