Polis Crusher — ruling 1

“Protection from enchantments” means that Polis Crusher can't be enchanted, it can't be targeted by Aura spells or by abilities of enchantments, all damage that would be dealt to it by enchantments is prevented, and it can't be blocked by enchantment creatures. Notably, it doesn't have protection from enchanted creatures (unless those creatures are also enchantments).

Polis Crusher — ruling 2

Polis Crusher must be monstrous as it deals combat damage to a player in order for its last ability to trigger. Once the ability triggers, it resolves even if Polis Crusher isn't on the battlefield at that time.

Polis Crusher — ruling 3

Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.

Polis Crusher — ruling 4

Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.

Polis Crusher — ruling 5

An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.

Polis Crusher : règles, fonctionnement et interactions (MTG)

Polis Crusher is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Trample, protection from enchantments : Monstrosity 3. (If this creature isn't monstrous, put three +1/+1 counters on it and it becomes monstrous.) Whenever this creature deals combat damage to a player, if this creature is monstrous, destroy target enchantment that player controls.

How it works : Polis Crusher

Original English text — Wizards of the Coast / Scryfall

#1September 15, 2013· Wizards of the Coast
“Protection from enchantments” means that Polis Crusher can't be enchanted, it can't be targeted by Aura spells or by abilities of enchantments, all damage that would be dealt to it by enchantments is prevented, and it can't be blocked by enchantment creatures. Notably, it doesn't have protection from enchanted creatures (unless those creatures are also enchantments).
No other card is named in this ruling.
#2September 15, 2013· Wizards of the Coast
Polis Crusher must be monstrous as it deals combat damage to a player in order for its last ability to trigger. Once the ability triggers, it resolves even if Polis Crusher isn't on the battlefield at that time.
No other card is named in this ruling.
#3September 15, 2013· Wizards of the Coast
Once a creature becomes monstrous, it can't become monstrous again. If the creature is already monstrous when the monstrosity ability resolves, nothing happens.
No other card is named in this ruling.
#4September 15, 2013· Wizards of the Coast
Monstrous isn't an ability that a creature has. It's just something true about that creature. If the creature stops being a creature or loses its abilities, it will continue to be monstrous.
No other card is named in this ruling.
#5September 15, 2013· Wizards of the Coast
An ability that triggers when a creature becomes monstrous won't trigger if that creature isn't on the battlefield when its monstrosity ability resolves.
No other card is named in this ruling.