Obliterators are VERY hard. Toughness 5, 2+ armour save, 5+ invulnerable, and two wounds. Reapers won't do it. Bright Lances will only do a wound a turn at best. Only strength 10 weapons will autokill and there aren't that many that can reach, and they still have a 1/3 chance of walking away. Without a doubt star cannons are best , but I don't want to be a player who takes nothing but Star Cannons. They are great weapons, but it seems unbalanced to me to take nothing but.
The problem with this particular army is if you are shooty you get a rhino and prince rush. If you are assault you tend to get blown to bits by the obliterators, then countercharged.
I went for the transports, knowing that would stop him choosing where to assault. That meant I took a lot of fire from the obliterators. None of his forces were spread out enough to choose an isolated target, so I assaulted one of his assault squads to stop them Furious Charging. I took a couple of turns to deal with them, but then the Obliterators turned up and held me while he repositioned his other sqauds. Guardians bounced off obliterators, D cannons couldn't get in range and wraithlord ended up being shot to bits by las cannons.
Now I could put together a force to take him out. Star cannons being the obvious way, but that's not the point. The point is that he has slaughtered his way through the store. He has found the weaknesses in the chaos Codex and exploited them expertly. And due to the complexity of the codex he can do that.