Aegar, the Freezing Flame

Aegar's ability won't trigger when a battle is dealt excess damage. It's not his fault. They didn't exist yet.

A planeswalker is dealt excess damage if it's dealt damage greater than its current loyalty.

It doesn't matter whether a Giant, Wizard, or spell you control deals the excess damage, only that excess damage was dealt and that one of those three things dealt damage to the creature or planeswalker at some point during the turn. For example, if a 4/4 creature an opponent controls is dealt 2 damage by a spell you control and later that turn is dealt 3 damage by a spell another player controls, Aegar's ability will trigger.