There are two bosses of concern, first Glacial which is the Trial's main boss and then its spawn the Glacons which will be summoned in a frequent basis and can overwhelm the party. Both of them work around the Ice Element and many of their attacks trigger as counter upon specific actions.
Regardless of the difficulty level the battle will always start with a complicated preemptive strike that must be survived and is by far the toughest hurdle of the battle.
All bosses have a -1000% Fire weakness and a +300% Ice and Water resistance and immune to all status ailments.
The preemptive strike will always be, summoning two Glacons which in turn use Crystal Lazer. This is followed by Glacial's White Reflector, Hail Powder, White Out and 4 normal attacks. For more information on the aforementioned abilities refer to each bosses' attacks.
Glacial has 44,444,444 HP in ELT difficulty but its DEF/SPR are astonishingly low.
As described above Glacial will always start the battle the same way. Cold air has formed a crystal, summons a Glacon and it will be used twice (on this and every turn). White Reflect is essentially Reflect to all the bosses. Hail Powder is a powerful (magic attack with physical damage) x10 damage multiplier attack that inflicts Ice/Wind damage on all units. However since this is self-casted on the enemy party and subsequently reflected, it will hit 3 units at random which due to Reflect mechanics, cannot be provoked or drawn. Followed by Whiteout which inflicts Paralysis and Disease with 100% success to all units and decreases Ice Resistance by 100%. Finally it will use 4 normal attacks.
Glacial will then repeat this pattern every turn, which summarizes a total of 9 actions per turn. It however has a handful of conditional attacks. If attacked with ability, attack or fire magic, it will use The melted ice is refreezing.... which decreases all damage taken by 100% to all the bosses for 1 turn, buffs their ATK/MAG by 50% for 3 turns, and finally restores 10,000,000 HP to all of them. Also on every 20% threshold it will use Frozen Hurricane which deals massive Ice/Wind damage to all units.
Glacial can be ATK/DEF/SPR Broken although it only has a 50% chance of success.
They always have 1/10 of Glacial's HP, that is 4,444,444 HP at ELT difficulty.
Their most basic attack is Crystal Lazer which is a decent strong non-elemental magic damage to one unit. If attacked with fire physical or magic, it uses The ice melted, the lazer is broken. Which is basically Crystal Lazer on all units. If attacked with ability or attack, then it uses Ice shards have stuck to the weapons. This decreases Ice Resistance to all units and enchants the Ice element to their weapons. The former lasts 99 turns the latter for 3 turns.
Glacons can be stat broken in all stats.
As listed above, the player's first and foremost priority is to survive the preemptive strike. Since this is an ambush, the player cannot buff themselves, so they are dependent on their equipment, Materia and Espers to do the job plus gear around SPR. They must also bring in Paralysis immunity and be able to survive the normal attacks and Magic Lazer.
All of this means the player will likely need a Magic Tank with Cover-esque abilities that can take the brunt of the attacks. Mystea is the ideal choice for this purpose. Paralysis immunity and removal will be helpful, a healer that has both suffices. Luka and Fina can do this although the former needs to be equipped with immunity.
The player can soft reset the app to get the desired result in which non-critical units are killed so as to guarantee a good outcome.
Following that ordeal, the rest becomes easier. Glacial has massive HP but its defenses are as very low and with a glaring -1000% Fire weakness it doesn't require very much ATK output to chain-kill in one turn. For reference two Agrias chaining with dual wield and a fire weapon can kill it one turn provided if: No DEF break: ~850-900 ATK ; 45% DEF break: ~650-700 ATK. As such buff/debuffs can make this easier but are not a total need at this point.
Killing it in one turn will prevent further Glacons from being spawned and they have a very small damage output all in all with just Crystal Lazer. Delaying their defeat is helpful to acquire Esper Orbs for the missions, however.
If Glacial cannot be destroyed in one turn, then the player will have to take Frozen Hurricane. Reflect and a Magic Tank and Ice/Wind Resistance can help with this. Reraise could be helpful too if really needed, however, the player should try their best to destroy the boss in one turn. As explained above a decent combo of chainers with Fire weapons should do the job fine.
For a much easier time in achieving the missions it is recommended to do them in two rounds, one focused in defeating it with 5 party members, the other for the Ifrit summon. It is worth noting that whoever the carrier of Ifrit is will have a -50% resistance to Ice so it should not be placed on a key unit. Garnet can help in making this mission easier.