I was pretty sure there would be a way, and you found it. I kinda dislike all the ifs: mana spring (can scan the save game for it, so no biggie), fewer elves & trap placement (those are pure luck and there is no way for you to know - you could play for hours just to discover it was for naught) - I wonder if you can cheat yourself into the shelter just to check how it looks, but the trap is still a big if, as you won't have the skill if you go peek at start... but... once you get through this, the game is pretty much won for you, so... Might as well be worth it. I had that trap exactly on the same spot as impy with the Demoness, and in one of the fights the Fauns squarely suicided on it, so I can confirm that a trap in that spot is extremely useful. I can also confirm that level 1 poison skull will do as long as you can force it to inflict over 100 damage by moving now and then.
Impy's analysis is based on the premise that every fight plays the same and the AI does the exact same thing, the only thing that changes the outcome of the battle is you. If you cast Poison Skull immediately at the start, or after moving a hex to any side, you will always get a different outcome and a different damage amount. The battle must also run in a perfect tick-tock rhythm - and if necessary, you need to write down what happens in each occasion, so you can adapt accordingly.
This is really high-grade tactical combat where your actions must not deviate by a hex. I doubt many people can do it, but it is doable. I also know how, but sometimes I get too frustrated to do it properly
