Alhena has 4,000,000 HP in HARD, is an Undead and Human enemy and like the other Bosses of the Chamber of Arms has multiple phases which are to be handled differently as the Boss shifts strategies and resistances. In both phases it has no elemental weaknesses or resistances, is immune to all status ailments, and is vulnerable to ATK/MAG Break.
The first phase last for the first half of his HP until reaching 49% at which point his HP won't drop further until the next turn. Every turn it will cast, It is conjuring powerful magic (Decreases Magic Damage to self by 50%), Blazing/Torrential Warhead (Fire physical damage (5x) to one enemy. Decrease fire resistance (50%) for 3 turns to one unit, Wind physical damage (5x) to one unit. Decrease wind resistance (50%) for 3 turns to one unit, respectively), Carnage Flare (Fire magic damage (6x) to all units), Tornado Cross (Wind magic damage (6x) to all units), Magic Fists (Hybrid damage (5x) to one unit), Eye of Light (Inflict blind and paralyze (100%) on one unit), Magnum Opus (Magic damage (10x) to all units).
During even turns it will use its Fire abilities, while in odd turns it will use its Wind abilities. Both are followed by Magic Fists used twice. Every 3 turns it will use Eye of Light once.
On threshold of 90% it uses Magnum Opus and ends its turn. On threshold of 70% it uses Magnum Opus and Eye of Light and ends its turn.
This phase takes the reminder of the fight. It casts It's melted together with magic power (Decrease physical damage taken (100%) for 1 turn to caster and cannot be removed) oncer and lasts for the rest of the fight. Luminous Ring (Inflict stop (100%) for 3 turns on one unit), The hole in the space is opening (Increase DEF/SPR (100%) for 3 turns to caster), Black Hole (Magic damage (10x) as MP drain (1000%) to all units), Flames of Anger (Fire magic damage (7x) to all units), Storm of Resentment (Wind magic damage (7x) to all units), Its resistance to fire and wind have been taken away (Decrease fire and wind resistance (100%) for 3 turns to all units), Arsmagna (Magic damage (13x, MAG) to all units). It no longer uses its previous moves.
During the transition it uses It's melted together with magic power, Arsmagna, The hole in the space is opening and it ends its turn. This is followed by Black Hole the next turn.
Alhena alternates between Flames of Anger and Storm of Resentment used once each turn, followed by Luminous Ring used once per turn and a maximum of 4 times only, he then follows with normal attacks.
At threshold of 40% it uses Arsmagna, The hole in the space is opening and ends turn. Following this it uses both Flames of Anger and Storm of Resentment once per turn each. At threshold of 20% it uses Arsmagna and The hole in space is opening and ends its turn. The turn after each threshold is always followed by Black Hole.
Difficulty-wise, Alhena is stronger than Elnath but less lethal than Sheratan, he will however require a bit more preparation than the latter. The player will need to keep its ATK/MAG broken all the time or his powerful attacks will likely wipe the party.
The player will need a Magic Cover Tank and a Provoke Tank both with as high as possible Fire and Wind resistance for optimum protection. Protection against status ailments is encouraged and during Phase 2, any means to prevent or remove Stop are encouraged. Mystea is a Magic Tank who can prevent Stop so she's ideal.
Elemental resistance buffs are ideal if the player cannot rise elemental resistance to 100% of both elements, which is not necessary but reaching as close as possible to this will ensure maximum defense. The Provoke Tank will have bigger issues because the physical attacks decrease elemental resistance so it will be at a disadvantage, although a handful of Provoke Tanks can also reduce damage to themselves, so with high enough elemental resistances and potent buffs they should be fine. Wilhelm and Veritas of the Earth are useful because of this.
For maximum damage the player will require both Physical Damage for Phase 1 and Magic Damage for Phase 2. Strictly speaking, however, only Magic Damage is really needed since Phase 1 doesn't not nullify Magic Damage, only halves its effectiveness, whereas Physical Damage will be useless in Phase 2. Magic units who can build damage up are recommended for the best results without requiring chainers. If the player has compatible Magic Chainers they can be fairly effective, especially with a finisher of any kind (even Bahamut's Megaflare which can easily end the fight if properly capped). Tornado chains are the best because of their long duration and the fact Alhena sets a weakness to Wind to himself, otherwise using Fire Magic can deal lots of damage. Firaja, the Emperor's Fire From Below or the Dragonlord's Kafrizzle will all deal enormous damage.
The player should be wary of the thresholds as Arsmagna will deal a lot of damage, enough to potentially kill either Mystea or Shylt, so Reraise will be needed for them to keep guarding the party in subsequent turns. Furthermore if the player cannot beat Alhena in one turn during Phase 2, they will be subject to a powerful MP Drain that will very much guarantee their main party will be devoid of any MP, the same is true if the player cannot beat Alhena after any threshold. As such the player will need powerful MP rechargers to instantly restore their MP back. Unlike Sheratan, Alhena only casts this after each threshold, so the player has more room to restore their MP to a comfortable level.
Ace and Blossom Sage Sakura are useful MP batteries, they are also powerful Magic units on their own right (the former will have to be enhanced for both roles, however). If they are used as main damage dealers it is recommended to swap them out before being subject to the MP Drain. Otherwise Lotus Mage Fina, Ayaka or Tilith can instantly restore HP and MP, Ayaka can also deal with Stop as well. Nonetheless it is recommended that the MP recharger is on the reserve while Alhena uses Black Hole and be able to restore MP instantly rather than as a Regen (Lotus Mage Fina's Manatopia is both an instant recharge and a Regen so she works).
If the player can deal with this annoyance then the battle is rather straightforward, as all the player will need to worry about is the temporal threat of Luminous Ring, and then deal with his elemental magic attacks which just requires high enough elemental resistance while the Provoke Tank must be resilient enough to withstand the physical attacks. The player can then attack Alhena at their leisure and heal/buff if needed. Eventually Alhena should fall.