For most Advent Ragnaroks, if your team is strong and you charge a Full Burst, you win. This makes Burst charging abilities extremely valuable.
Applying this to Rahab, one strategy is to stack HP and ATK down and just tank two Rage Overdrives; you'll need to dispel the ATK buff to avoid a wipe on the 2nd. Another strategy is to try and suppress damage (e.g., by holding DEF down) to let at least one normal Overdrive go off and give your team more time to charge, then tank one Rage Overdrive; again, you'll need to dispel the ATK buff from the first overdrive.
For Medusa, one strategy is layering 3-4 cleanses to stall as long as possible. Behemoth is 1, and Maiden's Prayer is 2. Mordred with Viccitudes of Fortune has fairly decent affliction resistance, making her a good candidate for Maiden's Prayer EX. Amon has an affliction protect, which can be useful for some teams. Another strategy is stacking so many damage abilities that the fight is over before Petrify can become an issue. But if you could do that, you probably wouldn't be here looking for advice.