The Demon of Resentment is a Trial in Final Fantasy Brave Exvius and was previously found on the Chamber of Arms before being relocated to Earth Tower in the Farplane. Although it was the "first" Trial of this location, it served as a predecessor for the true mechanics of further Trials on this location. Players face Erinyes.
Battle[]
Erinyes has 2,000,000 HP and can attack up to 7 times per turn. Erinyes will ambush the player with Eye of the Killer, targeting the unit with the highest MP and Stopping it for 2 turns.
Erinyes will after that, begin every turn by inflicting a status ailment to all units with 100% success each (rotation order is: Poison > Disease > Sleep > Blind > Paralyze > Silence > Confuse > Petrify > repeat from the start). It will also use Wall of Resentment or Physical Damage and Blind and Silence to one unit and cast Reflect to itself for 3 turns. It can use Beast Claw for Water and Light Magic Damage to the unit with the lowest HP.
Each 3 turns it will use Deep Tsunami for Water and Light Magic Damage to all units. Every 4 turns and its HP is below 50% it will use Evil spirits are all around, which enchants all units with Water, Light and Dark elements and buffs its ATK/DEF/MAG/SPR by 20% for 3 turns.
On HP thresholds of 70% will use Song of the Dead for Dark Magic damage and 30% ATK/DEF/MAG/SPR debuff to all units, and Fog of Conviction which casts Charm (works like Stop) on 1 unit. At 50% it will use Eye of the Killer, Song of the Dead and Evil spirits are all around. At 40% and 20% it will use Fog of Conviction, at 30% it will use Song of the Dead and end its turn.
Erinyes is weak to Fire and Lightning (-100%) and resists Water, Light and Dark (200%). It is vulnerable to SPR Breaks and is immune to all status ailments.
Strategy[]
The player cannot use companions for this battle which will make things tricky if the player needs a stronger unit or lacks some utility.
The first the player will have to do is account for status ailments which will befall on them every turn. They can cost the player turns, or even the battle depending on the severity so the player must immunize themselves to them as a priority. Equipment like Ribbon or Materia like Goddess' Protection can help but given they are Trust Master Rewards it will be unlikely to stock on multiples, so the player must resort to abilities.
Ling is an excellent choice as she can remove all status ailments with Dance of Life, although the player is still encouraged to immunize against Confusion at the very least so as to not endanger oneself. The Esper Lakshmi's Assuring Protection can help with that, although Equipment like Black Choker should suffice. A Luka with a fully enhanced Priestess' Miracle can remove every status ailment, although she will need to be immune to some herself. Other more complicated but still great units to use are Marie and Tilith, who both can provide elemental protection and status ailment protection/removal. However both are rare or limited, so Luka remains a more available option.
Stop will require Dispel-esque effects to remove them, so the player will require Dispel (Doublecast is needed to cast this oneself. If this proves too tricky Fingersnap is a much easier option but should be employed by a low ATK unit).
Erinyes has very high DEF so Physical Attacks while they can be used, won't be as effective unless the player can use high tier units and can build chains successfully. Fire weapons are common and they double as a weakness for Erinyes so those should be used. Otherwise her low SPR and SPR Break vulnerability encourage a much more traditional approach on Magic attacks. Exdeath, Emperor, Victoria are useful for this, the latter two have abilities that build up upon use which makes damaging Erinyes much easier. Imperils can also be used in conjunction with this for even greater damage outputs.
The player is also encouraged to bring on an MP Battery for this battle as MP will likely drain fast on this battle. Although many MP batteries are high rarity units, Charlotte with her enhances can provide major MP Regen, while being an effective Tank as well, she can provide solid survival options specially to protect Mages from the physical attacks.
A common alternative is to use Setzer, as his Double Dice ability will bypass the high DEF. It is encouraged that he's equipped with as many Demon Killers from as many sources as possible (Diabolos is dangerous as Erinys inflicts Light damage, but a proper elemental protection + equipment should compensate for this). Erinys' HP is modestly low for this tactic so chains are not a must but they can help.