Which enemies do you find most difficult to fight?

Gargantuas, Sorceresses, Djinns, Cyclops, Dark Knights.

Yes, of course, you listed some excellent monsters ;) They are all top-notch; it's especially scary to fight them, as they can destroy almost half your army. Allow me to add a couple more giants to this list:
1) Megadragons (Although they are not very common, they can still cause a lot of damage if they get close to your army :smile05:)
2) Cherubim
3) Peasants :D (Because there are usually almost 700 of them)