How challenging did you find the WRE campaign in BI? Set him up with that and see what happens :D

Alos, MTW2 will be horrifically easy as long as players can simply blitz their way to victory. This would obviously not have worked historically, and it's a very poor design that allows you to maintain an empire by sacking cities as fast as possible.

The player (representing the sovereign or the state) should get almost nothing from sacking a city. Sacking a city should raise dread, tank the cities infrastructure, decrease the population quite a bit, and make those troops be upkeep free for a few turns. You get virtually no gold to fund further conquests, because that is basically the way it worked historically. Commanders didn't routinely tell their troops to go forth and indiscriminately rape and pillage in the newly city, because that was often the exact opposite of what would be most economicallyhelpful for the empire. Instead they exterted much effort preventing such things from happening.

Also, players shouldn't be able to take an AI's city and sell all the buildings and then abandon it.

The AI is far too vulnerable to a scorched earth set of tactics, and such tactics are ahistorically rewarding to the side that adopts them.