The first ability of Porphyry Nodes doesn't target. You can destroy a creature with shroud or protection from white.

No player can take actions between the time you choose which creature to destroy and the time it's destroyed.

If one of the creatures with the least power has indestructible, you must destroy one without indestructible. If all creatures with the least power have indestructible, you don't destroy any creature.

If all creatures on the battlefield leave and immediately return (perhaps because Momentary Blink exiled and returned the only creature), the last ability of Porphyry Nodes triggers. You'll sacrifice it even though a creature's on the battlefield by the time the ability resolves.