The High Seraph is the Trial Level of the In the Name of Love event in Final Fantasy Brave Exvius. It is found in its own separate banner in the Vortex. The players face Ultima in her Arch Seraph form.
Ultima has 4,000,000 HP and will always have the first turn, ambushing the player with The power of darkness is coiling up! which adds the Dark element to the player's party physical attacks and inflicts Poison.
Ultima will mostly rely on normal attacks, being able to spam them up to seven times per turn, dealing moderate damage each, but as the battle progresses she gains other attacks.
Every 4 turns she will use Mute, which inflicts Magic Damage as MP Drain to the unit with the highest MP. This will most likely wipe the MP of that unit. Every 5 turns she will use Dispelja to remove buffs/debuffs and immediately reapply The power of darkness is coiling up! Every 6 turns she will use Grand Cross which inflicts Magic Damage and several status ailments to the party. On the 10th turn she will summon two Ultima Demons. This will be used along her Dispelja and Debuffs, furthermore the Demons will attack the turn they are summoned.
At the 90% and 50% HP thresholds she will use Eschaton, for major Magic Damage and end her turn. Every 2 turns after the 80% threshold she will use Ultima for great Magic Damage. The Ultima Spell will be followed by her normal attacks.
She has 100% resistance to Earth, Light and Dark, as well as immunity to all status ailments, except Blind (50% resistance) and immune to all Stat Breaks except MAG.
Although Ultima has high HP both her DEF and SPR values are pitiful. To compensate for that, the party will be repeatedly enchanted with the Dark Element on their physical attacks. However there are a handful of ways to circumvent this.
The player can use Dispelga or Bushido - Freedom (on themselves) removing the Dark Element (for the latter a unit with low ATK is recommended), then they will be able to attack normally. A second option is to mix weapons of other elements, preferably those not mitigated by Ultima as the damage calculation will average both elements thus resulting in Ultima taking damage. A third option is to use Magic Damage as it not subject to the Dark enchant, Hybrid Damage also works (provided another element is added). A pseudo-fourth option is to use elemental debuffs on Ultima to decrease her Dark resistance.
After taking in account any or all of the above, damaging Ultima will not be difficult. The difficulty of the battle however lies in being able to resist Ultima's deadly magical attacks and mitigate or heal the status ailments from Grand Cross. The former can be handled through SPR buffs, MAG Breaks and damage mitigation abilities like Defensive Barrier, available by Sakura and the recently released Veritas, while the latter can be healed with Esuna, Esunaga or mitigation equipment or skills (preferably against Paralysis and Sleep since they hinder actions).
A second layer of difficulty if attempting to get all missions or otherwise facing difficulty in damaging Ultima consistently, is found in the 10th turn onwards. Ultima will summon two Ultima Demons who will attack on the same turn with Nanoflare and Unholy Darkness for Magic Damage that can pile up quick. Furthermore given they are summoned on the 10th turn, Ultima will follow with a Dispelja, likely resulting in taking the full brunt of the attacks without any defensive buffs. This can be deadly but can be prevented, if the player can use Manufactured Nethecite or Sealing Blade as they will prevent the enemy from using a Spell once, so this will block Dispelja.
Nonetheless whether the battle was delayed in purpose or not, the player should attempt to finish Ultima quick after she summons the Demons as the three together can be difficult to withstand.
Fortunately for the player, Agrias who is available at full power for the event is more than capable of defeating Ultima. Her Divine Ruination although weaker than Orlandeau's is still strong enough to decently damage Ultima (provided the Dark element enchant is handled). Veritas of the Dark and Marie are more rare but still viable options to inflict major elemental debuffs on Ultima, making her much easier to damage.