Which monster do you like the most in Heroes 5 (level 6)?

But they have the ability to cast "Shattering Blade," with which they will kill any monster with any amount of HP, and they can be used to eliminate summoned Phoenixes with a huge amount of HP.

For these purposes, Messengers are still better - they are faster and more proactive, plus they can always be revived during battle.
And against a larger group, a Messenger is more effective; he removes all positive effects from the enemy, and you're happy.