There is a mistake in my example: "For instance, if you have a 10x10 puzzle with 6 2 in a line, you can safely draw (6 - 10/2) = 1 pixel from the mid of the line to both sides, plus (2 + 1) = 3 extra pixels to the left. The line would end like this: [ - o o o o o - - - - ]. I don't think I need to tell you to watch for the edge numbers that are easy to solve, right? Have fun!

Start marking X at zeroes, then go for the big numbers. The tip is: let H be half the number of lines (rounded down). For each number A bigger than H, draw (A - H) pixels from the mid pixel to both sides of the line. For instance, if you have a 10x10 puzzle with 8 in a line, you can safely draw (8 - 10/2) = 3 pixels from the mid of the line to both sides. The line would end like this:
[ - - o o o o o o - - ]
This technique can be used again any time you discard another line of the grid.
If you have multiple numbers in a line together with a big number, you can use the past technique and draw the sum of those numbers plus the quantity of extra numbers as extra pixels to the opposing side of the numbers. For instance, if you have a 10x10 puzzle with 6 - 2 in a line, you can safely draw (6 - 10/2) = 1 pixel from the mid of the line to both sides, plus (1 + 1) = 2 extra pixels to the left. The line would end like this:
[ - - o o o o - - - - ]

Strategy to win nightmare easily:
- Monk archer + another monk archer (load this one) + cleric
- Equip cleric with Mace of Restoration and both archers with +damage bows. Other equipments are optional.
- Aways level up the monks before the cleric. They need the bonus damage.
Plus to stick it.

Easy game. This step-by-step works even in extreme mode:
1. Close the airports and seaports from healthy cities.
2. Close the borders from Canada.
3. Give masks to the people until you research the vacine.
4. Vaccine the healthy cities to isolate the infected ones.
5. Wait while the people die, or try to save some souls.
6. Control the media or you may lose due to panic.
P.S.: If another city gets infected in the first day, restart the game.