Hmm... I've used Warhammer Fantasy Battles in this situation, but unless you have $150 on you that you don't really want, that's not a real option. That $150 would only get you the Dread Necro's army anyway.
In D&D rules, I'll have to third Heroes of Battle. There's always doing it statistically, too. Work out the chance of everything (Easier than it seems), open up an excel spreadsheet to do all the maths and insta-outcome. Then again, this isn't really that much fun