trucane: The problem with the monster guards is that the only one that can defeat them fairly easy at low levels is the wizard.
Also the monster guards totally removes the possibility to win quick games because the AI seems to be able to buy them in huge amounts early on turning the shard into a slow grind just to get heroes able to quickly kill the monsters.
jamotide: There are ways to defeat them with Scout and Warrior,too. They have been posted. Monster guards only pop up at late turns, so the way to avoid them would in fact be a "quick win".
Petrify should really be no problem, since it has a low resistance check. The AI is not smart enough to use spiders web effectively, this is a very powerful tool only for you to defeat Dragons or Cult of the Phoenix/Dragon. These things are hardly worth abandoning the whole game for.
But I know where you are coming from, as I said before I used to have much trouble with them, but now I learned to deal with it and they are nothing special anymore. The better you learn to play the game, the earlier you are ready to attack the AI, and the earlier you attack, the easier it is to defeat its guards.
Yes I know I can defeat them with the warrior/scout as well and I have done so in the past but a wizard at level 20 with some not that rare spells can easily do it but a warrior / scout at level 20 will have a lot more problems doing it especially without losing every single unit.
Also It's difficult to defeat the AI quick enough, I have seen monster guards very early from some of the masters and at that point all you can do is grind, grind and grind some more and that is certainly not an enjoyable aspect of the game
The whole problem is that guards overall are way tougher than the enemies heroes, it shouldn't be that way and weirdly enough the AI can always walk all over my best guards ( don't have monster guards yet so best one is patrolmen). Also the fact that the AI get guaranteed good alliances ( almost always with centaurs) within the 20 first turns is also ridiculous because it gives them strong and cheap guards early on.