Boss Monster expansion problem
When combining Boss Monster with Boss Monster: The Next Level you still use the standard hero allocations, i.e.


  Two Players: 13 Heroes, 8 Epic Heroes
  Three Players: 17 Heroes, 12 Epic Heroes
  Four Players: 25 Heroes, 16 Epic Heroes   


But you choose the hero sets from either two packs. For example, you can use the regular Heroes from Boss Monster, and the Epic Heroes from The Next Level, or vice-versa.

Heroes marked with a star can then be added on top of those numbers, or used in place of ones marked with the appropriate number of players (the star heroes are mostly promo ones, and are intended to make the game harder/more interesting).

You also combine each of the Spells, Rooms, and Bosses decks from both sets to form larger decks.