You lay out one path and the base tested version of the solo play is:
Give yourself 10 turns for easy, 9 turns for standard and 8 turns for hard. If you are not out of the forest in the turns you allot yourself then you are overrun and eaten.
Some people reduce each of those numbers by 1 if they find it too easy. Some also lay out 2-3 paths and allow themselves to choose which row to attack after each kill - adds more chance of varied cards.
Nightmare Forest: Alien Invasion - the stand alone sequel coming to Kickstarter Jan 24 ( www.tinyurl.com/2017NMF ) is similar to this game in some mechanics but is cooperative play style but will be able to be played solo as well - just some specific rule tweaks.
We tend to suggest for that solo play is you playing multiple characters but alone. Hope that makes sense