How do you start with the Boss Battles expansion, and where do its tactic cards come from?

Question

Accepted Answer

Each scenario from 21 to 30 unlocks a pack, and most packs contain tactic cards for the new monsters. Your first scenario can use base-game monsters, and the Unicorn or Cthulhu can be played in any scenario.

Keep the Heroes Out!

How do you start with the Boss Battles expansion, and where do its tactic cards come from?