Aegar, the Freezing Flame

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.

If a permanent is both a creature and a planeswalker, the minimum amount of damage to be considered lethal damage is used to determine if excess damage has been dealt. For example, if a 5/5 creature that's also a planeswalker with three loyalty counters on it is dealt 4 damage, it's been dealt 1 excess damage and Aegar's ability may trigger.