Broadside Bombardiers

• WOTC Official Reminder Textofficial rules
  Boast — Activate only if this creature attacked this turn and only once each turn.
• WOTC Official Reminder Textofficial rules
  Menace — This creature can't be blocked except by two or more creatures.

• WWizards of the Coast2023-11-10
  Official Ruling
  If it's not your turn and you gain control of a creature with a boast ability after that creature attacked, you can activate that creature's boast ability if it hasn't been activated yet that turn.
• WWizards of the Coast2023-11-10
  Official Ruling
  If an effect adds additional combat phases to a turn and a creature with a boast ability attacks more than once during that turn, its boast ability can still be activated only once.
• WWizards of the Coast2023-11-10
  Official Ruling
  The back face of a double-faced card doesn't have a mana cost. A double-faced permanent with its back face up has a mana value equal to the mana value of its front face.
• WWizards of the Coast2023-11-10
  Official Ruling
  If a creature with a boast ability is put onto the battlefield attacking, it was never declared as an attacker. Its boast ability can't be activated that turn.
• WWizards of the Coast2023-11-10
  Official Ruling
  A boast ability can be activated at any point after the creature with that ability has been declared as an attacker. This can be before blockers are declared, after blockers are declared but before combat damage is dealt, during combat after combat damage is dealt, during the postcombat main phase, during the end step, or, in some unusual cases, during the cleanup step.

Rulings via Scryfall