The Showdown is a quest in Chapter 17, "Deliverance from Chaos" in Final Fantasy VII Remake. After defeating Jenova Dreamweaver, Cloud Strife runs outside to chase after Sephiroth.
Walkthrough[]
Run outside the door on the right of the office and use the bench to heal, then head around to the left and use a ladder, commencing a cutscene. Approach the helipad to start a cutscene and a boss battle against Rufus and Darkstar, fought solo by Cloud. Ensure Cloud has 
Healing Materia, and Time Materia can also be useful. The boss has no elemental weaknesses, but pairing 
Elemental Materia with Lightning Materia on armor to provide resistance to some attacks can be useful, and Poison Materia makes for good damage over time, as Rufus in the second stage of battle is harder to hit. 
Steadfast Block Materia is also useful for how often guarding will be required as it further reduces damage taken and builds up Cloud's ATB faster, and Parry Materia allows Cloud to remain mobile even in Punisher Mode.
Rufus boss[]
Boss battle.
Rufus will use a shotgun attack that stuns Cloud, while Darkstar can use Corkscrew. Guard against attacks frequently, before laying into Darkstar with Cloud's attacks under Punisher Mode to pressure it. Cloud's Counterstance ability is extremely useful for dealing damage. Darkstar will heal Rufus if attacked, so it is best to defeat the Darkstar first. Use fast counter abilities that are more difficult to dodge, such as Triple Slash. After Darkstar loses 50% HP, Rufus will gain a new powerful ranged attack, Bright Lights, that must be dodged instead of guarded against.
Once Darkstar is defeated, focus on Rufus. Rufus will follow up most of Cloud's attacks with a counterattack, so he should be guarded against (or dodged in the case of Guns Akimbo and Bright Lights) until he uses Reload. When he does, use attacks against him and fast character abilities, such as Focused Thrust. Because Rufus is very mobile, Cloud should stay in Operator Mode. Counterstance can be used often to get in more damage, and after Rufus uses Bright Lights, he is vulnerable to magic spells. Limit breaks can easily stagger him when used, at which point Cloud can use powerful abilities such as Infinity's End to defeat him.
Rufus is instantly staggered if Cloud's Braver connects to him. The player can then use Stop on him to lock the stagger state in for an easy win with Infinity's End. The player can hit Rufus with Braver by using the skill with impeccable timing, or by putting him to sleep first.
After Rufus is defeated, a cutscene commences, following which the next quest, "The Great Escape", begins.
Hard mode tips[]
A similar materia setup works as in normal mode, though the greater number of materia slots allows for more flexibility. Cloud can be given 
Chakra Materia if running low on MP, though since this is the final quest in the chapter involving Cloud, there is no issue spending as much MP as is necessary. Transference Module is very useful to allow Cloud to use limit breaks faster. As spells are less useful for this battle, Cloud should use 
Hardedge to optimize physical damage.
As before, Counterstance is reliable for most offensive potential in the first portion of the fight. Casting 
Regen is the most MP-efficient way to cure Cloud, and as before, lightning + elemental materia can allow Cloud to absorb the Thunderclap attack. In the second phase, wait for Rufus to reload to attack. Using Ascension can allow Cloud to end the fight in this phase.
The same Sleep+Braver+Stop+Infinity's End tactic still works.
Defeating Rufus earns Cloud's The Art of Swordplay Vol. XIV.