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.

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.