Council of Elrond

Rank Upon Rank:

All non-agent Man attacks receive +1 prowess and +1 strikes. If Doors of Night is in play, all Giant attacks also receive these bonuses. Discard this card when such an affected attack (automatic, hazard creature, or otherwise) is defeated. Cannot be duplicated.

Rank Upon Rank is in play, and Assassin is played. Is Rank Upon Rank discarded if just one of the three attacks is defeated, or only if all three attacks are defeated (i.e. the creature is defeated)?

Reading it sounds like I only need to defeat an "affected attack" not an "affected creature" (which may create multiple attacks), but want to make sure I'm not getting off the hook too easily.

Yes. Rank Upon Rank would be discarded when any affected attack is defeated -- any attack of an Assassin.