You can find the rules for monsters in the grand alliance books for the factions, like Agents of Chaos. The monsters get some abilities that help them deal with terrain. A tripple that lets you drag an enemy fighter within 6 inches to within 1 inch of your monster. A quad can demolish a whole terrain feature, everyone falls to the floor, suffers impact damage and then you remove that terrain feature. I haven't actually played with monsters yet, but I can imagine how these abilities could help.
I finished the first campaigns of all factions in about 10 hours and it was pretty fun. The harder difficulties are more of the same, still fun but it does start to get a bit grindy, because you have to find good gear and skills to be able to win some harder battles. But I absolutely loved the voice acting in the story missions, especially the nighthaunt and nurgle. I wish there was more depth to the whole campaign progression, where taking a specific path would actually matter. Overall I think its a pretty good aos game, if you are a fan of any of the factions you should try it out