Here's a rundown of Garuda Ragnarok, modified from what's shown on the JP wiki by using my Damage Calculator to normalize the damage numbers to completely unmitigated.
Normal attack: ~2700 damage
Special attack: Hits 3 random targets for ~3300 damage each
Normal Overdrive: ~8300 damage to entire party
Raging Overdrive: ~11,000 damage to a single target
Garuda Ragnarok doesn't appear to self-buff, however she hits hard and her overdrives are very dangerous. Black Propaganda is a must; neither massive AoE damage nor single-target KO can be endured multiple times in succession. This is the time to abuse ATK down stacking. Jeanne (damage mitigation), Andromeda (healing, Revive), or Mordred (Black Propaganda, debuff stability) are recommended for this fight.
This fight could be extremely difficult. Simply keeping your entire team alive until the first Burst may be very challenging, even for those with maxed teams.