"Pandemonium is a harsh realm, and its inhabitants must be resourceful to survive. But none more so than the scavengers, who armor themselves in the very rocks beneath their feet. They may appear as mighty giants to their foes, but a few hard strikes will reveal that their true form is much smaller."— Deckard Cain(src)
Primordial Scavengers have two forms in which they can be encountered.
In their non-combat form, they take full damage from all attacks, move very quickly, and cannot attack. However, they will retreat and begin to burrow into the ground, which will take them roughly 4-6 seconds. If not killed quickly, they will take their combat form, becoming encased in rocks.
In combat form, they are much slower, but their melee attacks hit very hard, and they block most conventional forms of harm directed at them, effectively becoming highly resistant to damage done by players. After absorbing a certain amount of damage, the Primordial Scavenger's rock armor will shatter, and the beast will revert to non-combat form again. The cycle can repeat infinitely.
The main diffculty is killing them while in the weaker state. This can be complicated if there are other enemies in the area, as Scavengers tend to run off as far as possible when weakened.