Shemhazai is fought during a regular playthrough of the game and needs to be defeated in the Ancient City of Giruvegan after leaving the Great Crystal.
Shemhazai is located in the last room of the Ancient City of Giruvegan referred to as the Gate of Wind.
Level: 25
HP: 91,136
Steal: 
Sagittarius Gem (55%), 
Hi-Ether (10%), High Arcana (3%)
Shemhazai’s attacks are entirely focused on Magick. She will hit you with an abundance of elemental attacks, so keep 
Shell active for the duration of the fight. You should also equip a 
Rose Corsage on your healer to prevent them from being Silenced and set up an Esuna Gambit to remove it from your other party members. Shemhazai will use 
Silencega several times throughout the fight.
Set up a Dispel Gambit to remove the beneficial status effects from her at the beginning of the fight (she starts off pre-buffed with the spell).
Shemhazai’s regular attack has a chance of inflicting the Disease status effect. Set up a Gambit to use 
Serums on any party members that are affected by the status as the Esuna spell will not be able to remove it. Her other attacks do Magick-based damage, for a fair amount of HP, but they are not too difficult to deal with.
A video of the battle in the Zodiac Age version is included below:
Congratulations – you now have Shemhazai as an Esper which you can use in battle!
Shemhazai has two attacks that do not have an elemental affiliation. The first attack is called Devour Soul which works very similar to a 
Knot of Rust.
The second attack is also non-elemental and deals damage to all foes within range which is equal to the sum of all Knot of Rust damage done plus all of the Devour Soul damage ever done divided by 3.
Unlocking Shemhazai on the License Boards of certain jobs will unlock additional License Boards:
Dragon Helm, 
Dragon Mail), Heavy Armor 11 (Magepower Shishak, Maximillian), Heavy Armor 12 (Grand Helm, Grand Armor)Steel Mask, Mirror Mail)
Caldera, Volcano)Cleanse, Esuna)
Spica, Antares), Guns 6 (Arcturus, Fomalhaut)The optimal choice is quite often to give Shemhazai to your Archer so that they can wear some of the stronger Heavy Armor in the game. Alternatively, you can give your Red Battlemage the White Magick spells if they do not have them already.
