Seraphic Greatsword's triggered ability won't trigger if the equipped creature attacks a planeswalker controlled by the player with the most life.

Once the triggered ability triggers, it doesn't matter what happens to players' life totals before the ability resolves. You'll create an Angel token even if the player the equipped creature attacked doesn't have the most life as the ability resolves.

Although the Angel token created by the triggered ability is attacking, it was never declared as an attacking creature (for the purposes of abilities that trigger whenever a creature attacks, for example).