The Trial offers different difficulties from ADV to ELT. ADV and PRO should be considered warm-ups for the real challenge in ELT as the Dark Espers power will skyrocket and the true rewards earned here.
The player faces both Bosses at the same time and their tactics often involve teaming up. Their attacks cover different elements although Dark is a common element used by both and they will also slightly reduce the player's resistance to Dark during the first turn (they attack preemptively).
The course of the battle changes depending on who the player fells first, regardless whoever is left standing will have increased power and restore its HP fully. If Dark Ifrit is defeated first, the player is left to deal with Siren who will focus in harassing the player with status ailments and MP drain on top of greater damage. If Siren is defeated first, the player will be left to deal with Ifrit who will have much greater offensive abilities. Trying to defeat both at the same times is unwise as they have attacks that trigger upon both reaching a certain threshold which can be dangerous.
As her Preemptive move she will increase ATK/MAG/DEF/SPR (50%) for 5 turns to both herself and Dark Ifrit.
In her normal state, Dark Siren will Osmose to player inflicting damage and draining the MP of a single unit (has a chance at 50% every turn and then once every 3 turns). Water/Dark Magic to a single target or to the whole party (30% chance both every turn).
Once possessed (when Dark Ifrit is defeated) she will self-buff +100% ATK/MAG + 300% Fire Resist + Full heal. Subsequently she will still attack and drain MP, use Raging Hellfire for Fire/Water/Dark magic damage to one target or Overwhelming Hellfire for Fire/Water/Dark magic damage to the whole party.
Her threshold attacks (70% / 50% / 30% HP) include a +50% ATK/MAG, +100% DEF/SPR buff and her strongest, Ultimate Voice the next turn for great (x4) Water/Dark Magic AoE with 75% sleep. Dark Siren can be dispelled through the usual means, or through inflicting Water or Fire damage on her. If fire damage is used Ultimate Voice will change its effects to even larger (x6) Fire/Wind Magic Damage to the party and no Sleep.
If possessed Ultimate Voice will do major (8x) Fire/Water/Dark Magic AoE with 100% sleep, but if hit with fire it will be modified with the usual Fire/Wind Damage attack.
If inflicted with Blind she will do a 50% ATK/MAG self-buff, status removal, 5x Water/Dark Magic AoE with 100% Sleep/Silence.
If Dark Ifrit is dropped past 50%, Dark Siren performs additional attacks. If she's dropped past two thresholds she will heal both herself and Dark Ifrit. She will heal by 100,000 to both if she's dropped to 70% and 50%, but if instead dropped from 50% to below 30% she will full-heal both.
Dark Siren is vulnerable to Fire (-50%) and resistant to Water, Ice and Dark (+200%). She's immune to all status ailments except Blind (50% resistance) and is susceptible to DEF/SPR Breaks.
As a Preemptive Attack he will Decrease Dark resistance (10%) for 5 turns to the party.
In his normal state he will deal 2x Fire/Dark Magic to the party (30% chance at any turn and then every 3 turns after). Fire/Dark Magic to a single target (30% chance every turn) and a stronger physical attack (x1.7) on a single target (30% chance every turn).
Once possessed (when Dark Siren is defeated) he will use +100% DEF/SPR + 300% Water resist + Full heal + AoE -30% ATK/DEF/MAG/SPR debuff for 3 turns. Great (x4.5) Fire/Water/Dark Magic damage to the whole party (60% chance every round). Greater (x5) Fire/Water/Dark Magic to a single target (50% chance every round). And a stronger (x3) version of his Physical Attack (50% chance every turn).
His threshold attacks (70% / 50% / 30% HP) include a + 50% ATK/MAG, +100% DEF/SPR buff and his strongest attack which deals Large (x3.8) Fire/Dark Magic to the party in normal state and a massive (x8) Fire/Dark/Water version when possessed. His attack won't change regardless of the element used, although Water will dispel him, albeit only once.
Trigger attacks include a 50% ATK/MAG buff if his DEF/SPR is debuffed. 100% MAG buff if hit with Fire. Massive (x9.99) Physical Damage to the party if dropped past two thresholds at once, and (x9.99) Fire/Water/Dark Magic Damage to the party if he's dropped below 50% while Siren is below 50% and alive.
Dark Ifrit is vulnerable to Ice (-50%) and resistant to Fire and Dark (+200%) resists all status ailments and is vulnerable to ATK/DEF/SPR Breaks.
The battle is more tricky than difficult. With the proper setup, the battle will become rudimentary and a repetitive cycle of actions that the player can prepare for every turn. By far the most annoying part of the battle is trying to summon Siren and Ifrit in order to complete one of the Missions.
A Tank should be used to keep (mostly Dark Ifrit's) damage on check, this will help achieve the No Party Member KO mission. Cecil and Snow are useful, the former has Curaja and a major buff in his Limit Burst and Snow is a major counter user which can be used to fill in LB gauge if needed and charge the Esper Gauge. Cecil can be equipped with a Black Belt to make him capable of Counter.
A buffer to keep the Elemental Damage at bay. Marie and Minfilia are useful. Marie can increase resistance by 50% to all elements with Love You All, inflict Elemental Debuff and Dualcast magic to use Protectga and Shellga. Minfilia has the benefit she can increase resistance to all elements by 70% although she can only do so one at a time. This makes her more useful in the long term after three turns, but will make the party more vulnerable during the first turns. Marie can buff Fire and Water by 70% with Green Magic but has no way to cover Dark outside Love You All.
A healer to keep the party alive. Y'shtola or Refia (with Dualcast) are useful. The former comes equipped with innate Dualcast and Green Magic, the latter has Full-raise and status ailment immunities although if one tries to not die, Full-Raise is not much of a benefit, although Refia has Embolden which is a useful buff.
For attackers the player should have preferably good chainers that double as damage dealers in order to amass Esper Orbs while dealing reasonable damage to the Bosses. Orlandeau is a good option as his attacks circumvent the Bosses' resistance provided it has no weapon of an element they are resistant to. A Holy weapon works best with his Divine Ruination, and he can also use Crush Weapon to weaken their offensive abilities. Fryevia can do good damage to Dark Ifrit but is useless against Dark Siren making her deadweight so she's unwise. Setzer is also a competent chainer and can deal fixed damage with his Dice and Double Dice abilities although the former is unreliable and the latter attacks all enemies which can be dangerous.
Chainers should be combined if possible with damage cappers. Olive, Gilgamesh, Luneth, Noctis provide solid options. Noctis also doubles as a utilitarian unit, although Warp Break should not be used on Ifrit as it can trigger self-buffs, instead Ice Flask can deal with him.
Other units of worth are Ling whose debuffs can severely decrease their ATK/MAG and DEF/SPR so one can navigate through the Bosses' gimmicks. She also doubles as a MP battery and overall great support unit. More recently Warrior of Light at his maximum can serve as an effective Tank and his debuffs are as powerful as Ling's and last longer.
As stated above, the course of the battle changes depending on whom the player defeats first. Starters should defeat Dark Ifrit first, as his higher damage outputs can be a problem, whereas Dark Siren's MP drain while annoying can be mitigated through a Tank provoking her and using MP healers. Ultimately it depends on the player's ability to survive damage or deal with MP absorption.
The Bosses' attacks are nothing too complicated and rather simplistic. As a result the battle (if following the above recommendations) can be summarized in: Cycling through Buffs (elemental protection and stat augmenters), Dispel/Debuff the Espers every so often (specially on thresholds), provoking Dark Ifrit with a Tank, Healing and finally attacking the espers (focusing in one Esper at a time).