Major thing with Garuda story mode is killing the plumes fast so they don't kill the rocks which are needed to absorb the ultimate.

If the healer is struggling to top everyone up after the ultimate, get them or BRD if you have in party, to use the healer LB to get everones HP back up for the final phase. Also never stand in front of Garuda. As there a part where she does a really nasty frontal AE that can kill. When she TPs back towhere she started, everyone should move as far away from her as possible whilst staying within the inner circle.

Shame you aren't on Odin as I could have helped.