We're currently playing the original campaign, but with monsters, heroes, classes, etc from all expansions except the two big ones. The biggest difference this makes, it seems, is that the Overlord is able to throw in far more open groups with massive monsters, given the availability of them for all terrain types. We're often facing giants, ettins, shadow dragons, golems, chaos beasts, and often several at the same time, meaning they're blocking up terrain and gives him better reinforcements. We've played several encounters where there were no 1x1 monsters on the map.
My question is, have other experienced that this unbalances the campaign, since it wasn't written or playtested for such a great availability of Big Ones? Are there any strategies to counter it effectively? We've often found that we hacked down a master giant, only for him to reinforce next turn and be back in the same spot, blocking the same path, before we got to move anywhere.