Operation: Save Aerith is a quest in Chapter 16, "The Belly of the Beast" in Final Fantasy VII Remake, taking place in the Shinra Building. After overhearing a board meeting, Cloud Strife, Tifa Lockhart, and Barret Wallace follow Professor Hojo to find Aerith Gainsborough.
Walkthrough[]
Follow Tifa and Barret and leave the Conference Area, leading to a short cutscene, after which the party arrives in the Laboratory Passage. Head forward up a flight of stairs to a room with a vending machine, a bench, and a treasure chest containing two antidotes. To the left is a chest containing two vials of celeris, and to the right is a corridor used to proceed. Follow the path to see Hojo, and follow him up the stairs, after which a cutscene begins. The next boss is not weak to any element, but resists 
Lightning. 
Cleansing Materia is useful to equip, as is materia or equipment providing resistance to 
Poison. Equipping Healing Materia is crucial, which can be paired with 
Magnify Materia to heal all allies; alternatively, magnify materia can be paired with damaging materia, such as a Fire Materia, to hit all enemies. Buffing materia, such as the Time Materia, can also be useful.
Specimen H0512 boss[]
Specimen H0512 attacks.
A boss battle commences against Specimen H0512, as well as its H0512-OPT minions. The enemies are not weak to anything, and initially do not pose a great threat; Specimen H0512 will use simple melee attacks at first. Cripple its left claw using character abilities to pressure it, then lay into it with abilities and spells. Tifa's Starshower is an effective way of dealing damage while avoiding damage herself, and Barret can use abilities such as Maximum Fury at a distance.
After it is down to 3/4 of its HP, Specimen H0512 will become slightly more powerful, and also summon H0512-OPTα. These are similar enemies but more deadly in melee range, meaning they can be defeated with spells using magnify or with ranged attacks. The strategy is the same as before, however; cripple Specimen H0512's left claw to pressure it and then deal damage.
Once it is down to 1/4 of its HP, it can use Mako Expulsion, a ranged attack, and is capable of lunging throughout the battlefield in powerful melee attacks. Guard against these attacks, and cripple its left claw once again. Defeating it will earn a talisman.
After boss battle[]
Once it is defeated, follow Hojo and take the elevator up to start a cutscene. After this, elite riot troopers, elite grenadiers, and elite security officers appear; the latter can be dealt with using character abilities, while the former are best dealt with using spells, or certain abilities that can overwhelm their shield.
Armored shock troopers arrive.
Once the enemies are defeated, a cutscene commences, and armored shock troopers are fought. They can deal considerable damage with their sword and machine gun. Use character abilities to stagger them, then deal as much damage as possible, using Tifa's Unbridled Strength martial techniques to increase the damage they receive. After they lose HP, they can use Eject, causing an explosion and replacing them with enhanced shock troopers. These enemies are less durable, but are quick, and when using Twister, will repel melee attackers. Use Barret's Overcharge or abilities to pressure them, then have Cloud attack in Punisher Mode to damage them further to stagger.
Once all enemies are defeated, a cutscene commences. Chase after Aerith and another cutscene starts. After this, as Cloud, approach the elevator to start another scene leading to the next quest, "A Way Out", in the chapter "Deliverance from Chaos".
Hard mode tips[]
During the Specimen H0512 battle, a similar setup may be used as before, though pairing magnify with time or barrier materia may be preferable to pairing it with healing materia, in favor of using 
Prayer Materia and being less MP-reliant. Elemental Materia can be paired with fire materia and equipped on armor to absorb the smaller enemies' fire attacks. However, Warding Materia may also be important to pair with Poison Materia to shorten poison's length. If Cloud is casting spells they should be paired with HP Absorption Materia to restore HP; Tifa can pair HP Absorption materia with 
Deadly Dodge Materia to heal herself when attacking the smaller enemies.
The main difficulty will come from the final phase, when Poison and 
Slow are inflicted on the entire party. With time + magnify, it is possible to cast 
Haste twice, first to remove slow, and then to hasten the entire party; alternatively, with barrier + magnify, using Manawall on all can make the party more resilient. Beyond this, remain at a distance with Barret, using Lifesaver and curing him with Regen and Curaga, while using Pray to cure the entire party. Other than this, the tactics are the same as before: wait until after Specimen H0512's attacks to inflict attacks from the party to pressure and eventually stagger it, then unleash all powerful attacks.
Defeating Specimen H0512 will grant Cloud The Art of Swordplay Vol. XII. After this, against normal Shinra soldiers, fire + elemental is an effective combo.