Death Frenzy

• WWizards of the Coast2014-09-20
  Official Ruling
  Only creatures on the battlefield when Death Frenzy resolves will get -2/-2. However, if a creature enters the battlefield and then dies later that turn, you’ll gain 1 life for it.
• WWizards of the Coast2014-09-20
  Official Ruling
  You gain 1 life for each creature that Death Frenzy causes to have a toughness of 0 or less. The delayed triggered ability is created before state-based actions cause any creatures with 0 or less toughness to die.

Rulings via Scryfall