Parry is better to use on low health creatures that deal high damage. The creatures I usually use as an example are Grim Reapers. Using an elemental damage charm will only do 195 damage. 215 damage if you use the Divine Wrath, Enflame, or Zap charm as they are 10% weak to those elements.
Parry will do equal damage to the creature that is dealt to you when it triggers. Grim Reapers have many strong attacks and some that can even deal 500-800 damage.
Another case where Parry could be useful is if the creature is immune or strong to the damage types of any of the charms you have available. When Parry triggers it deals "neutral" or "true" damage back to the creature. The damage dealt back has no element so will not be reduced.