After casting the commander as soon as possible, it is important to ensure Avacyn stays on the battlefield. The various swords that provide protection from colours help with this, but some cards are dedicated to protecting creatures or the whole boardstate.
INSTANTS
Cloudshift
Ephemerate
Mother of Runes
Teferi's Protection
Cloudshift and Ephemerate have the additional utility of flickering creatures with useful triggers on entering the battlefield. Teferi’s Protection is universally a good protection card. Mother of Runes is useful in instances when Avacyn is being targeted in response to attaching Equipment, or to make Avacyn unblockable.
AURA
Hyena Umbra
Hyena Umbra is the only Aura in the deck because for one white mana it can protect Avacyn from being destroyed, adds +1 power and gives her first strike, which is helpful against potential blockers. Overall a good value for one mana.