Bahamut cannot be called to fight against Spathi, so he will sit out this one. Ifrit or Ixion are the best of the rest, and preferably one or both has a full Overdrive gauge. Spathi will not attack but starts a countdown until he uses Mega Flare, at which point Shielding is recommended.
Spathi is Greek for "sword". Likewise, Spathi's original name, "Tsurugi", is Japanese for "sword".