Sudden Attack is a quest in Chapter 4, "Mad Dash", in Final Fantasy VII Remake, taking place in the Sector 7 Slums. After Cloud Strife settles in for the day, he is awoken by Tifa Lockhart, who reports an attack by mysterious monsters.
Tifa joins the party, and several Mysterious Spectres will appear. These enemies move slowly, and resist physical attacks. Magic spells are best against them, as they can pressure them. Cloud should have magic materia equipped while Tifa can either equip spells and cast magic, or simply use her martial techniques with Unbridled Strength to deal optimal damage.
After the first group is defeated, follow the marker to Seventh Heaven. After running into more monsters, Tifa will direct Cloud through another route; follow her and a group of ghosts will fly past Cloud, restricting his movement. Move forward to reach Seventh Heaven, where another battle begins.
Tifa begins the battle bound by an Enigmatic Spectre; use a spell or some attacks from Cloud to break her free. The Enigmatic Spectre resists damage until pressured, by destroying a smaller Mysterious Spectre. Destroy one, and then focus down the Enigmatic Spectre with spells or normal attacks. This can eventually stagger the Enigmatic Spectre. Once staggered, use Tifa's Unbridled Strength with a martial technique to increase the damage it takes during stagger, and then lay into it. Repeat the same tactics until the Enigmatic Spectre is destroyed. If no Mysterious Spectres are available, simply attack the Enigmatic Spectre with spells until some are. Later in the battle, the Enigmatic Spectre can create a black hole, trapping them; simply keep dodging the attacks until this ends.
Once the Enigmatic Spectre is defeated, a cutscene commences, and the battle ends. Barret will encourage Cloud to sort out gear before moving to the next mission. After leaving Seventh Heaven, Tifa is waiting outside. On the way to the station, consider visiting Chadley to earn materia from any Battle Intel objectives, and the weapon store for any armor and accessories for Barret. Upgrading weapons is also a good idea for preparation, and the music disc "Barret's Theme" can be purchased from the Item Vendor in a stall.
At the Sector 7 Undercity Station, talk to Barret, and select "Yes" to get on the train for the next quest, "All Aboard for Sector 4".
Hard mode tips
The Mysterious Spectres can be defeated with character abilities from Cloud and Tifa, while the Enigmatic Spectre is more challenging. The player needs to stagger a Mysterious Spectre with low level magic and attacks like Focused Thrust/Focused Strike to pressure the Enigmatic Spectre. When the boss is staggered, Tifa's abilities and Cloud's Infinity's End can defeat it. The party can expend all MP here if needed.