Challenging Advent Ragnaroks

Advent Ragnaroks are balanced around a maxed neutral-element team.
Your attack power as listed on the main screen should be around 40k, and your +ATK from weapon skills should be at +100% / avg SLVL 15. Less than that and your damage output and HP will likely be lacking--unless you have elemental advantage.

Debuffs and utility skills are critical.
You’ll need a source of overdrive gauge extension (e.g., Mordred’s Black Propaganda) and encounter-specific utility skills such as cleanse (Maiden’s Prayer) and dispel (Chaos Magic). In general, you’ll also need strong debuffs: aim for -30% or more ATK and DEF down. Blind and combo attack down are also handy.

Healing is useful.
You’ll likely need at least one healer--unless your damage output is especially high.

SSR kamihime will increase your damage potential
Many fights are a race against the Rage bar. If you can’t deplete it in time, you’ll take massive damage that will likely overwhelm even highly defensive teams. SSR kamihime have big bonuses to Burst damage and damage skills that will help you deplete the Rage bar, ideally before the 3rd overdrive.