The team would benefit from both Sol and SSR Nike due to the ATK and DEF down they provide, respectively. Water doesn't need Gaia because it gets Snow Raphael, and covering your weak element actually isn't that important in the big scheme of things. Assume you'll get Snow Raphael with your 2nd ticket and plan accordingly. That'll put you at -35% ATK and -35% DEF base, or -50% DEF if you equip Ambush as EX. Subbing Sol for Aphrodite or Poseidon would max ATK down, while subbing Nike would free up an EX slot, which is important if you don't want to use Mordred. Dragon King is also an option if you want to focus on offense over defense later.