If I remember correctly, this is very much like implementing RTS AI-type files. That is difficult. RTS gamers frequently complain about the lack of challenges presented by the AI, and the AI (artificial intelligence, if you don\'t know already) can be easily defeated by old-time gamers. I assume that you are asking for monsters using tactics, and that would require an AI, which is rather difficult to make challenging. Also, some may be challenging at first, but can be effectively and easily countered later. An annoying tactic the AI in AoK always used with \"shoot-and-scoot.\"
I myself would find intelligent enemies more challenging (and exciting). It is best if it is not overdone, though, as in another game, \"Wyvern,\" there is a creature known as a... Wyvern. It breathes fire. Sure, it\'s overused, but it is incredibly deadly. So deadly that my rather powerful Dwarf (I think) always ran away when he saw one. And Firedrakes too, for that matter, because both creatures could breathe fire. The majority of my later deaths were caused by fire-breathing creatures.