Nope never played insane mode. But from reading posts here you will find that the monsters are only tougher. Increased hit points and they save alot more.
They hit alot more and do more damage as well. I haven't been disappointed with IWD/HoW. I like it better than BG2 actually. The linear flow and not having morons asking you to do things they should do for themselves all the time is a nice change. Do you have Heart of Winter?
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