(Gen 9) National Dex Monotype Choice Specs (Steel)

Aegislash @ Choice Specs
Ability: Stance Change
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
IVs: 0 Atk
- Shadow Ball
- Flash Cannon
- Steel Beam
- Air Slash / Hidden Power [Ice]
Despite being outclassed by the string cheese Steel/Ghost type known as Gholdengo, Aegislash can be used on Steel teams as a Choice Specs Pokemon that serves as a nice offensive progress maker that can trade against opposing Pokemon thanks to its Ability Stance Change. Shadow Ball gives Aegislash a Ghost STAB to use against Pokemon that resist its Steel STABs, being useful in matchups such as Water and the mirror matchup of Steel(especially Steel since it doesn't really have a Ghost resist outside of Bisharp, which isn't really used on Steel teams.) Flash Cannon and Steel Beam on the same set may seem weird, but there are reasons for this. Steel Beam provides Aegislash with a powerful Steel STAB that lets it trade down with opposing Pokemon. For example, Specs Steel Beam can OHKO Gliscor (provided it doesn't Knock Off your Choice Specs) and has a good chance of OHKOing a +1 SpD Kommo-o since Kommo-o runs Naive Nature in National Dex Monotype.
252+ SpA Choice Specs Aegislash-Both Steel Beam vs. 244 HP / 236 SpD Gliscor: 382-451 (108.5 - 128.1%) -- guaranteed OHKO
252+ SpA Choice Specs Aegislash-Both Steel Beam vs. +1 0 HP / 0- SpD Kommo-o: 283-334 (97.2 - 114.7%) -- 87.5% chance to OHKO
Flash Cannon provides Aegislash with a spammable Steel STAB since it doesn't take away half of its HP unlike Steel Beam. Despite not seeming like it does much, Air Slash is nice as the last slot on Choice Specs Aegislash since it allows Aegislash to have a solid chance of OHKOing Ogerpon-Wellspring as well as being able to OHKO Buzzwole without having to use Steel Beam in order to do so. Hidden Power Ice is an option for the last slot to be able to OHKO Ground-types such as Gliscor, the Landorus formes, and Garchomp/Mega Garchomp without having to use Steel Beam.
The EV spread for this set can be customized to your preference. You can go a bulky Specs route, a fast Specs route, or just enough EVs to reach specific speed benchmarks. The one I'm providing on this set is a Max HP Bulky set since it allows Aegislash to survive a Clangorus Soulblaze into a +1 Flamethrower from Kommo-o at full HP so you can then have a chance to kamikaze the dangerous Steelkiller Clanger Dragon with Steel Beam. (unless you get REALLY unlucky with the sequence such as one of the moves critting, getting highrolled by both moves, and missing Steel Beam)
252 SpA Kommo-o Clangorous Soulblaze vs. 252 HP / 4 SpD Aegislash-Both: 93-111 (28.7 - 34.2%) -- 2.6% chance to 3HKO
+1 252 SpA Kommo-o Flamethrower vs. 252 HP / 4 SpD Aegislash-Both: 182-216 (56.1 - 66.6%) -- guaranteed 2HKO
Note: Aegislash-Both is used in the damage calcs just so I didn't have to switch between the offensive and defensive forms for calc purposes.