Tbh I think this is too simple to bother writing a guide about. All that needs understanding is the concept of what chance you have to get one of the featured monsters you don’t own. You can gather this concept from the guide I’ve already written.
I’ve written the simple tip at the top of the thread since the egg changes long ago. It says that you should go for eggs with lots of featured legendaries and ones which you don’t already own. The only bit not included there is that you should stop hatching once you get one, or maybe two, of these featured legendaries because your odds of getting one of the remaining ones drop. That’s not really a big leap to make. However, I might re-word it a little to make this clearer.
EDIT: Just re-written the top bit. It now reads…
NOTE: This thread is out of date since changes to the egg system in December 2018. Now all featured monsters are 10x chance and there is no guaranteed in the 6th pack.
The best eggs are typically those with 1.5x legendary drop rate and with a large number of limited legendaries and mythics, featuring monsters you don’t already have. Once you hatch one or two of the featured monsters you don’t own then the chance of getting a new featured legendary tends to drop drastically, so it’s a good time to stop.
The following thread will be left untouched as a mathematical piece that may be relevant later.