A monstrous creature is so huge and powerful that its attacks make a mockery of armour; foes are not so much wounded as hideously mangled and crushed.
A monstrous model (Carnifex, Greater Daemon, etc) ignores Armour Saves.
When attacking vehicles, a monstrous model rolls 2D6 for Armour Penetration and adds them before adding their Strength.
Monstrous creatures are treated just like independent characters in an assault and always count as being a unit in their own right.