Quite frankly the VH campaign setting makes the game much easier. Diplomacy is a waste of time, so you just blitz and swarm the AI nations under. My first campaign was on VH/VH, and I tricked the French into a marriage alliance early on... which they broke about 4 turns later, destroying their reputation and leading to their eventual excommunication. Ten turns later, there was no France.

In my M/VH England game I made the decision not to blitz, and France still exists in 1304, as do almost all the nations.

If you want to play full bore slash and burn you probably won't notice a difference between easy and very hard campaign difficulties, because the primary change is in AI aggressiveness, and the player's slash and burn is always going to be far, far more aggressive than the AI.

I haven't experimented with the different battle difficulties, I just left it on VH, but if the AI were to sally less on lower difficulties I imagine they might actually be considerably more difficult.