Water
Primary eidolon: Garuda. Garuda and Jormungandr synergize well, yet maxed Jorm friend eidolons are common while maxed Garudas are not. By picking Garuda, I improve my chances of getting the eidolon combo I want (2x Jorm is approximately a 20% DPS loss).
Andromeda: 1600 cap healing on a 5 turn CD, type A +20% def buff, Resurrection. I'd equip Sniper Shot (type B -20% atk/def) and the SR Lance from the Water Disaster.
Belphegor (Gatcha SR): type B -12% atk/def debuff and +12% atk/def buff, greatly reduce rage bar + damage, extend stun. Excellent utility toolset, especially against boss mobs.
Triton (Gatcha SR): combo attack down debuff, type A -15% atk debuff + damage, type C 15% atk/def up buff and self-heal.
Gabriel (Event SR): type A -12% def debuff + damage, type B +12% atk/def buff to entire group
Nike (Story SR): 750 HP heal, type A +12% def buff
During your typical Ultimate advent boss you'd be able to stack -45% atk (!) with Jormungandr as sub- or friend eidolon, -32% def, and combo attack down, and due to overlap from Snipe Shot and Belphegor's skill you'll be able to re-apply if one misses. Plenty of heals from Andromeda and Nike, and a defensive buff of 12-32% can be active at all times for total damage mitigation of up to 58% (!!!). Excellent against bosses due to Belphegor's rage reduction skill and both stacking and staggered attack buffs, plus the stun extension. And the team is full Water so you'll be able to take full advantage of your assault skill. In addition, you can equip Apocalypse as a sub-eidolon for Blind and Typhon to buff your Bursts even further.
Note: Aphrodite's regen only applies to herself.