I don't consider myself an expert player of Arkham Horror. Or at least, I don't consider my usual group as an expert group. We played five games over the course of Friday and Saturday night and ended up winning 3 of them. For me to consider myself an expert I reckon we would, at the very least, have needed to win 4 of them if not all of them.
But ... even *we* find Yig to be too easy.
He was the Ancient One in our last game and before he woke up we were struggling throughout. We didn't really have any decent fighters (we always do a random selection of two characters each then pick one of those two to use) and every monster that came out had a minimum toughness of 3 (with the exception of the Cultists who were all dealt with by Wendy Adams and her handcuffs - you have to wonder how lame these Cutlists are that they let a 10 year old girl arrest them *g*). So with all these monsters running around town and with The Dark Pharaoh Herald hurting us whenever we tried to find an Elder Sign, it was tricky. So eventually, Yig woke up. And we chewed him up and spat him out.
After we killed him, we decided to keep going and see how many successes we could get before he finally killed us all. We got 96 successes before the last of our four Investigators died.
Given this, I can see why some people say that killing the Ancient One counts as a draw and not a win. But then again when we went up against Yibb T'Stll with a team of expert fighters we were absolutely massacred - there is no way beating Yibb T'Stll in a fight should be considered a draw, it should be considered a miracle! 
So my question is this ... What's the best way of making someone like Yig harder but not making Yibb T'Stll harder? The best way I can see is to select one of the Black Goat Of The Woods difficulty cards AFTER you've selected an Ancient One. If you see you have Yig or someone else really easy, use the ultra hard Difficulty card. The one problem with that, though, is that the wondering around town section of last night's game was hard enough as it was. We really didn't need to make it harder. Just Yig ...
? That's kinda lame.
.