While my group has been playing Talisman, I've been paying attention to threads that can help me gauge the difficulties of my encounters in the different Expansions. My First expansion was Frostmarch and most recently the Dragon. When I saw the Deck Percentages thread by Charlie Day (http://boardgamegeek.com/thread/737254/deck-statisticspercen...), I saw an opportunity to base Lethality on Enemy Card draw Percentages.
This alone wasn't enough to predict the Lethality level of the encounters, so I added an Avg Str/Craft stat and an ecology of Enemy types to overlay.
After adding The Frostmarch, I felt the game was harder. I was surprised to find that based off my Lethality scores, The Frostmarch has the Lowest Lethality score of all of my Expansions (at 3.0, the main game has a 3.7 rating).
As we started in the Dungeon, I was impressed with the Lethality of the monsters on average there (~4.65). Since adding the Dungeon, we have always included it on top of any small expansions available.
With the most recent addition of The Dragon, our players saw their highest Stat Pools ever. Not a single player had under a 10 in their primary stat when the game was over and no Casualties occurred.
Normally, at least 1-2 characters die and no one gets above 12 in a stat in our games. After going the Lethality rating of the Dragon (5.0 Str, 5.4 Craft) I realize Lethality might not be the best way of describing the difficulty of the Expansions. We were dying more often in a 3.0 Lethality than a 5.4!
My realization, and something I want peer verified, is that this is a game of randomness (!)
What I've been gauging isn't a measure of the difficulty of the encounters, but more of the average Trophy Value of any successful encounters. While a higher value might result in more lost battles at the beginning of the game, it also means that stats will be leveling up faster as well. This will actually decrease the time to Auto-Kill status, where everything in the game can be defeated without rolling the dice.
A lower Trophy Value, conversely, means that the result will be based off the dice rolls more often. A player will have to accumulate more trophies to level up per point leveled. Keeping the player at a lower stat over more encounters actually increases the Total Lethality of the game!
Don't take my word for it. Play with Frostmarch only and let me know if more deaths occur in your games (not just the Basilisk!)