All non-player enemies have 90% Critical Severity and 50% Critical Strike. Critical Avoidance does not reduce an enemy's Critical Strike, but rather directly decreases the enemy's Critical Severity. To determine how much extra damage an enemy will deal on a critical hit, the enemy will deal 90% (enemy Critical Severity) - Character's Critical Avoidance Total Percentage times as much damage on a critical hit.
For example, say a character has 50% Critical Avoidance and an enemy is attacking them for 100 damage. If this attack critically hits, the player will take: 100 damage * (1 + (90% - 50%)) = 140 damage