Lena, Selfless Champion

• WWizards of the Coast2018-07-13
  Official Ruling
  Lena's last ability affects only creatures you control with the appropriate power at the time it resolves. Creatures you begin to control later in the turn won't gain indestructible, and a creature you control whose power increases later in the turn won't lose indestructible.
• WWizards of the Coast2018-07-13
  Official Ruling
  The number of nontoken creatures you control is counted only as Lena's first ability resolves. If Lena is still on the battlefield (and not somehow a token), it will count itself.
• WWizards of the Coast2018-07-13
  Official Ruling
  To determine which creatures gain indestructible while resolving Lena's last ability, use Lena's power as it last existed on the battlefield.

Rulings via Scryfall