Now I will get to what he did in a minute, because it was not all that revolutionary, and is something that you can easily replicate. I have to give kudos to him first, because he does something that a lot of people don't. They play their army, and their list exclusively. He doesn't jump around and play a ton of lists, or change his unpainted marines to the latest codex. In fact he plays his Chaos Marines, and keeps to it. He must have hundreds of games in with his list, and it probably only slightly varies for his interest.
Playing a single list with small variants (to improve it) is something most people do not do. They do not play their lists enough times to get all the nuances of it down, and so that you can see it against almost any variety of lists out there. Normally the first time you face a list type, it is very hard, but once you have (practice makes perfect), the game becomes much easier and you do better.
What he was fielding was a mostly Infantry army with Autocannons and plasma guns with infiltrators. Of Course there was some Terminators, a Vindicator, and some rhinos, but most of his units were on foot. His Terminators were 10 strong, 5 of which were in area terrain, and the other 5 strung out of the area terrain so that he could give cover to his big pieces of armour. Those terminators were big enough.
From another area terrain piece, a set of ruins, he had strung his guys outwards, and placed units behind them, with his Autocannons getting perfect shots behind the lines of defense. His two Demon Princes where behind the ruins, and unless I sitting back there with them, could not see those buggers at all.
Then he went along, and infiltrated right into area terrain in other places of the board, stretching his plasma gunners out so that they could get into range.
Now the cool thing about this, was that I was looking at not a single model in his entire army that would not get a cover save. The only shot I had that could help me here, was my basilisks, and of course the first shot scattered off the table. One entire round of shooting from my 2500pt IG shooting army killed off 2 marines, and 3 terminators. Granted he made some good saves, and went to ground with one unit, but I was amazed at how feeble my entire shooting line was.
Next time I encounter something like this though, I will be taking pictures of how it was done. It would be much better than me trying to describe it.
Needless to say, practicing your deployment, in order to get the most of it, is imperative. I have played far too many people that lose the game at deployment. (I have done this myself).