Mathas, Fiend Seeker

• WOTC Official Reminder Textofficial rules
  Menace — This creature can't be blocked except by two or more creatures.

• WWizards of the Coast2017-08-25
  Official Ruling
  The opponents who each draw a card and gain 2 life are the opponents of the player who controlled the creature as it died. This may not be the player who controlled the creature when it received a bounty counter.
• WWizards of the Coast2017-08-25
  Official Ruling
  The target creature that gets a bounty counter still has the triggered ability it gains even if Mathas leaves the battlefield.
• WWizards of the Coast2017-08-25
  Official Ruling
  If Mathas's ability resolves two separate times targeting the same creature, that creature will have two instances of the triggered ability. Each triggers separately when that creature dies.
• WWizards of the Coast2017-08-25
  Official Ruling
  Creatures that receive a bounty counter other than from Mathas's ability resolving won't gain the triggered ability. Creatures that receive more than one bounty counter as Mathas's ability resolves (most likely due to Doubling Season) won't have the ability multiple times.

Rulings via Scryfall