http://daemons40k.blogspot.ca/2012/08/how-you-beat-dem-orks.html
This article is written from the POV of the daemon player. The author states that orks are one of the hardest armies for daemons to beat. Granted, this article was written before the new codex came out so take it with a grain of salt. I re-read it with 6th edition in mind and found most of it to still hold true. While the daemon army has changed since the article was written, orks have remained the same.
I hope you find it helpful.