Most mass removal spells don't have any targets or at least don't target any creatures, so hexproof creatures will fall to them. Hexproof only stops targeting from opponents. The nly things that target are spells and abilities on the stack that explicitly identify the objects and/or players they are going to affect by using the word "target". Anything else is not targeted. There is no such thing as indirect or implicit targeting in Magic.
In Garruk's Wake does not use the word "target", therefore it does not target, and so hexproof is useless against it.