I use a set mechanism for increasing difficulty based on the number of power cards claimed. And its organised in a way that makes it easy to remember when you need to increase difficulty.
You start on EASY (not STARTER) and then increase the difficulty every time you get to a certain depth in the power up deck. Its simple to set up… Just halve the deck and then halve one half again; ie. three packets 50%, 25% and 25% approximately.
Turn over the top card on each of the smaller packets.
Stack the packets into a single deck with the 50% packet on top.
Whenever you reveal a face up card or finish the deck, then you increase the difficulty.
I often play solo with a one or two characters. This approach tends to work for me to keep things a little more challenging. Often the game will end on Difficult or Heroic, which leaves Epic for Dangerous foes only (ie. those that increase difficulty). But I imagine it should work with larger parties as more characters burn through more power-ups more quickly.
The approach works on the first three volumes and is easy to adjust if you need to make things more or less challenging by adjusting the number of times you split the deck; for example, for a more challenging game divide the deck into 4 quarters and have a face-up card on the bottom three quarters.
Having difficulty increase in line with power ups helps to keep the game a bit more balanced and helps maintain tension in the game. And forcing you to increase difficulty at a time not of your choosing makes a lot more sense to me; its too easy to forget entirely, or postpone difficulty increases till “after the next boss”.