The Ambitious Engineer and the Darkening Cloud

Abilities

 * Protect: Increase DEF (999%) for 99 turns to one ally.
 * Shell: Increase SPR (999%) for 99 turns to one ally.
 * Haste: Increase number of attacks to X per turn.
 * Gatling Gun: Magic damage (2.2x) to all enemies.

Conditional attacks

 * First turn: Protect, Shell, end turn.
 * Every 3 turns: Protect, Shell.
 * 50% HP threshold (Max 1/battle): Haste, Gatling Gun, end turn.

Regular attacks

 * Up to 4 attacks per turn before haste.
 * Up to 9 attacks per turn after haste.
 * Normal attack.

Abilities

 * Protect: Increase DEF (999%) for 99 turns to one ally.
 * Shell: Increase SPR (999%) for 99 turns to one ally.
 * Haste: Increase number of attacks to 12 per turn.
 * Anti-Magick: Reflect all spells for 3 turns to self.
 * Gatling Gun: Magic damage (2.2x) to all enemies.
 * Gatling Gun: Magic damage (2.5x) to all enemies.
 * S-27 Tokamak: Magic damage (2.8x) to all enemies.
 * S-27 Tokamak: Magic damage (3.3x) to all enemies.
 * S-85 Cyclotrone: Magic damage (3.3x) to all enemies.
 * S-85 Cyclotrone: Magic damage (3.6x) to all enemies.

Preemptive strike

 * Protect, Shell, Anti-Magick, end turn.

Conditional attacks

 * When Famfrit dies: Haste. All abilities receives damage upgrade.
 * Every 4 turns and Famfrit is still alive: Protect, Shell.
 * Every 5 turns: Readies tokamak. Next turn use S-27 Tokamak, end turn.
 * Every 6 turns (Chance to use): Gatling Gun.
 * 90% HP threshold (Max 1/battle): S-85 Cyclotrone, end turn.
 * 60% HP threshold (Max 1/battle): S-85 Cyclotrone, end turn.
 * 30% HP threshold (Max 1/battle): S-85 Cyclotrone, end turn.

Regular attacks

 * Up to 8 attacks per turn before haste.
 * Up to 12 attacks per turn after haste.
 * Normal attack.

Abilities

 * Briny Cannonade: Water magic damage (6x) to one enemy
 * Waterja: Water magic damage (4x) to all enemies

Preemptive strike

 * Readies cannonade, end turn. Next turn use Briny Cannonade, end turn.

Conditional attacks

 * When Doctor Cid dies: Briny Cannonade.
 * Every turn and Doctor Cid is alive: Readies cannonade, end turn. Next turn use Briny Cannonade, end turn.
 * This cannonade will target the lowest HP unit.
 * Every turn and Doctor Cid is dead: Briny Cannonade.
 * Every 3 turns and Doctor Cid is dead: Waterja, end turn.
 * 70% HP threshold (Max 1/battle): Waterja, end turn.
 * 50% HP threshold (Max 1/battle): Waterja, end turn.
 * 40% HP threshold (Max 1/battle): Waterja, end turn.

Regular attacks

 * Can only use regular attack if Doctor Cid is dead, otherwise it'll keep readying or using Briny Cannonade.
 * Up to 3 attacks per turn.
 * Normal attack.

Strategy

 * Depleting Famrit's MP makes him stop using water attacks, which makes the 2nd part of the battle - piece of cake. Use Lance, Duskblade or other MP Draining skills. (Note: He will resume using Briny Cannonade after Cid dies regardless of MP)
 * Bring Xon and use Twist of Fate to steal Doctor Cid's 999% DEF/SPR buff. Enjoy invulnerability.
 * Active DEF buff is capped at 300%, while there is none for SPR, so you will notice much less DEF increase.
 * Copied buff has separate turn duration from the original, it has 2 turns duration.
 * Otherwise, bring water resistance buff and dispel.
 * Tidus has non-magic (for 'No Green Magic' mission) 100% water resistance buff.
 * Minfilia, Luka, Seabreeze Dark Fina, Beach Time Fina, Summer Lid has non-magic 70% water resistance buff.
 * Marie has non-magic 50% water resistance buff and non-magic 40% imperil all elements debuff.
 * Rikku has non-magic 40% water resistance buff & 30% party wide damage mitigation in the same skill.
 * Tilith has non-magic 40% water resistance buff, and a non-magic dispel ability to remove his invulnerability. Also, a 40% all stat buff.
 * Equipping Siren gives 50% water resistance.
 * combining Siren water resist 50% with 70% water buff Aqua Guard will make a character immune. If you put this on the lowest hp character then Famfrits water attack Briny Cannonade (while cid is alive) will do zero damage.
 * Doctor Cid is vulnerable to Man-Eater while Famfrit is vulnerable to Demon Killer.
 * Use Diabolos to get both.
 * Sun Barrette gives Demon Killer.
 * Elza is the only unit with both Man-Eater and Demon Killer by default Firion can enhance his Legend Killer to damage Demons and Humans at +1.
 * Doctor Cid has high resistance to all elements except light and dark. If you are not using any imperil effect don't use elemental damage or weapon.
 * Orlandeau's Divine Ruination debuffs light resistance by 50% effectively reducing Doctor Cid to 0% and Famfrit to 50%.
 * Both Doctor Cid and Famfrit can be charmed.
 * Double check the weapons on all your units, including your friend unit, to make sure nobody is using weapons with ANY elemental effect, (except dark) because all Cid has 100% elemental resistance for all elements except light and dark, which he has 50% for, and Famfrit has 200% for water, 100% for light.