Angels and paladins – how to use them? There are also discussions about them here.

Good speed and initiative, they deal good damage, and with the support of light magic, they will become a truly formidable force. Thanks to light, they will often move first, hit harder, and take less damage. The opponent simply won't be able to do much... In general, Haven excels in massive, fast attacks, especially when the hero is well-developed in magic.
P.S. In the late game, Haven will show everyone its strength.

There are plenty of other units with good speed and initiative... Why should they now be considered "invincible"? And a truly well-developed mage can be either a wizard, a dark elf, or a necromancer, but definitely not a knight (rare exceptions are often found in the form of structures on the map that increase knowledge and magic power). Of course, this isn't a problem – you can buy one of them in the tavern and level them up, but in my opinion, the Haven army should be commanded by a knight; I think that's more beneficial.