Infiltration and Bust Out of the Garage are the first quests in Chapter 16, "The Belly of the Beast" in Final Fantasy VII Remake. Cloud Strife, Tifa Lockhart, and Barret Wallace attempt to enter the Shinra Building.
It is possible to revisit these quests on chapter selection to defeat a large number of enemies quickly and level up 
Materia easily.
Walkthrough[]
Follow Barret and Tifa down a set of stairs and ladders. When the two stop, wait for the dialogue to end, then head further down the stairs to a linear bridge. On the side is a treasure chest with two hi-potions, and at the end is a treasure chest with a turbo ether next to a vending machine which sells previously available items. To proceed, from the vending machine, head left and down the stairs to a linear path, and a cutscene begins. After this, the group jumps onto a Shinra track and enters the underground parking lot.
After the cutscene, the group enters the parking garage and fights several elite security officers in the parking lot. These can be defeated with simple character abilities, and area-of-effect abilities such as Cloud's Triple Slash and Blade Burst, Tifa's Divekick, and Barret's Smackdown can simplify this. After defeating them, the fight continues further into the parking lot against more elite security officers along with elite grenadiers. Once all are defeated, continue left through the garage to reach a gate labeled "Gate 0-1". Open it to commence a fight with more troopers, along with 3-C SOLDIER Operators, which are more durable enemies but can still be defeated with abilities.
Once the enemies within Gate-01 are defeated, open Gate-02 on the other and head straight into a side room with several boxes. Defeat the troopers and guard dogs here with character abilities, grab the treasure chest with a Molotov cocktail on the right, and proceed left then leave the room on the other side through the gap under a gate and proceed forward. Alongside previously fought enemies, elite riot troopers are fought here, against which, magic spells are the simplest way of defeating them, though characters can also simply attack them from the rear while they focus on another character, or wait for them to lunge forward and expose themselves. Some abilities, such as Barret's Maximum Fury or Tifa's Overpower, can also overwhelm riot troopers and cause them to drop their shields, and other abilities can also hit them around their shield.
Once the enemies are defeated, a door leads inside the main building. Head inside, then take the stairs up to reach an entrance, after which a cutscene commences, leading to the next quest, "Shinra Building Intel".
Hard mode tips[]
In the interest of preserving MP, spells should not be cast. 
Fire Materia and 
Elemental Materia can be paired effectively on weapons to maximize damage against the enemies here. 
Deadly Dodge Materia will also be helpful to deal area-of-effect damage to multiple enemies, especially on Tifa, and 
HP Absorption Materia will provide greater sustainability while doing so. Fire materia can also be paired with Magnify Materia to hit all enemies at once, though it should either be used sparingly, or a second fire materia should be equipped on the same character with MP Absorption Materia to minimize cost.
Beyond this, Barret can use Lifesaver on many of the fights to protect the party, and 
Regen cast on him should be sufficient. 
Prayer Materia can be used for any emergency healing, and First Strike Materia allows battles to be begun favorably, especially with a fire + magnify combo.
Grinding[]
This chapter is one of the best places for level and AP grinding after beating the game and unlocking Chapter Select. By clearing the enemy waves at the parking garage and restarting the chapter, the player can accrue a vast amount of experience and AP, specially given that those values are increased after finishing the game once. It is recommended to do this on the Easy mode, since enemy experience and AP values are unaffected by difficulty levels.
By equipping Cloud with a combination of Mythril Saber, 
Fire Materia paired with Magnify Materia, as well the First Strike Materia, the player can one-shot enemies easily by spamming Fire or Fira. The ATB Stagger Materia can also be equipped to further enable Cloud's damage, as enemies will often be instantly staggered upon being hit with fire spells.
Other party members can be equipped with AP Up Materia (paired with a materia of choosing, preferably one that requires a lot of AP to maximize, such as Revival Materia), EXP Up Materia, and Gil Up Materia to further speed the process.
Considering that characters outside the main party also gain experience, the player doesn't need to worry about leveling Aerith if they follow these steps.