Serra Ascendant

• WOTC Official Reminder Textofficial rules
  Lifelink — Damage dealt by this creature also causes you to gain that much life.

• WWizards of the Coast2017-11-17
  Official Ruling
  If Serra Ascendant is dealt damage at the same time that you gain life (most likely because it dealt combat damage to a creature blocking or blocked by it), and your life total becomes 30 or greater, apply its last effect before considering whether or not the damage dealt to it is lethal.
• WWizards of the Coast2017-11-17
  Official Ruling
  In a Two-Headed Giant game, anything that cares about your life total checks your team’s life total. Serra Ascendant gets +5/+5 and has flying as long as your team has 30 or more life.

Rulings via Scryfall