It is a random pull of characters/levels/skills. I have finished 15/16 but one day out of my last 4 nodes I had 3 high level Poe hiding glass cannons in a row and just lost a toon each battle. Since the update I have noticed that the levels and skills are slightly higher, but the overall difficulty is about the same. However, remember every now and then you can get a couple bad pulls and lose some toons. Depth is important and having a toon or 2 to sacrifice can be super helpful.