Not sure if this will be helpful to anyone so late in the event, but I was finally able to conquer Siegfried without native Elarra. My best general healer is still Y'shtola BSB, so I am always dancing around having strong but inefficient heals. Water Chain damage output was more than enough, but took about a dozen attempts to get timing and team placement right for survival.
Slot 1+2 have Damage+ / Slot 3 has Sap Resist / Slot 4+5 have Interrupt Resist
Tidus & Yuffie have 2x Water Boost equipment / Rikku has 1x Water Boost weapon.
Refresh Godwall once you have gauge, when Y'shtola looses TGM, you'll need both healers up and running.
Moving Tyro to the Slot 3 gave one extra SB gauge for a Keeper's Tome after second Godwall. Unsure if Magic Blink helped here.