My group only ever got him down to about 25% before it disbanded and I stopped playing. There are a lot of guilds who can stomp all over every other boss in there but still haven't beaten Yogg. He's brutal.

The clouds thing always seems to be a problem for someone. Out of probably close to 100 tries from our group, we only ever had a perfect first phase once or twice. Sometimes it was simply a problem with lag. If you have any kind of lag at all, you can't see where the clouds actually are, just where they were a few seconds ago. It's really screwy. I've watched group members just blithely walk through cloud after cloud, and they thought they were standing in the clear.

When you can see the clouds, though ... they're really not that hard to avoid, as long as you remember that avoiding them is more important than anything else you might be doing at the moment.