Brash Taunter

• WOTC Official Reminder Textofficial rules
  Indestructible — Effects that say "destroy" don't destroy this permanent. A creature with indestructible can't be destroyed by damage.
• WOTC Official Reminder Textofficial rules
  Fight — Each of those creatures deals damage equal to its power to the other.

• WWizards of the Coast2020-06-23
  Official Ruling
  If the target creature is an illegal target when Brash Taunter's last ability tries to resolve, the ability doesn't resolve. If Brash Taunter is no longer on the battlefield, the target creature won't deal or be dealt damage.
• WWizards of the Coast2020-06-23
  Official Ruling
  If your life total is brought to 0 or less at the same time that Brash Taunter is dealt damage, you lose the game before its middle ability goes on the stack.
• WWizards of the Coast2020-06-23
  Official Ruling
  A creature can be dealt an amount of damage greater than its toughness. For example, if Brash Taunter is dealt 3 damage, its middle ability deals 3 damage, not 1, to the target opponent.

Rulings via Scryfall