most annoying creature to kill

[Minecontrol]

Demoness (switching your vulnerable archer troop with their tank)
Shaman
Catapult (you have to take them on immediately as fire is more dangerous than it was in legend)
Goblins (their speed and the way they become hasted when attacked)
Fairies (damage is magical so stoneskin will not help if you use Divine Armor there are also ones which use dispel and DA is an expensive spell)

Ancient Vampires for me were very awkward as i had very high critical chance, there were a few battles where i was forced to use damage spells to thin out a stack of Ancient Vampires as none of my troops (except Red Dragon Fireflood ability) could injure them due to Deaths' Evasion.

Giants are always annoying to fight too, helplessness helps to bring them down quick however.