The difficulty of personal story battles appears to be currently based on total party size, versus the number of people in the instance. I just attempted Armaments (Norn story) for about an hour. My friend and I were in a party together, but he was off questing elsewhere. The quest seemed overly punishing and I died so many times I went from full repairs to naked. He came to join me and the number of mobs were exactly the same as when I ran it alone. In a single attempt, we easily beat it.
I have observed other story instances like this, where if I am partied but the party member(s) is/aren’t in the encounter, the fight feels very, very difficult. Dropping party or attempting it with two people make the fight suddenly manageable.