I’m not sure if this breaks the rules or not, but I’ll go ahead and answer.
It wouldn’t be that great.
Let’s take a look at it’s matchup against some of NatDex AG’s top threats.
(All calculations give Serperior the Dragon typing, so it gets STAB)
An incredibly potent sweeper in the metagame. It’s speed is higher than Serperior’s, and it’s special attack….
252 SpA Life Orb Calyrex-Shadow Astral Barrage vs. 0 HP / 0 SpD Serperior: 320-376 (109.9 - 129.2%) -- guaranteed OHKO
252 SpA Choice Specs Serperior Draco Meteor vs. 0 HP / 4 SpD Calyrex-Shadow: 220-259 (64.5 - 75.9%) -- guaranteed 2HKO
And here’s a display of how Serperior’s SpA…isn’t great. Even with specs and STAB.
Another huge threat in the metagame. Runs Specially Defensive, Physically Offensive, or Specially Offensive sets. Isn’t quite as fast as Serperior….
252 SpA Choice Specs Serperior Draco Meteor vs. 0 HP / 0- SpD Yveltal: 250-295 (63.6 - 75%) -- guaranteed 2HKO
252 SpA Choice Specs Serperior Draco Meteor vs. 252 HP / 252+ SpD Yveltal: 160-190 (35 - 41.6%) -- guaranteed 3HKO
But can take a hit regardless. But can it hit back hard enough?
252+ SpA Life Orb Yveltal Oblivion Wing vs. 0 HP / 0 SpD Serperior: 398-468 (136.7 - 160.8%) -- guaranteed OHKO
Offensive Yveltal can.
0 Atk Dark Aura Yveltal Knock Off (97.5 BP) vs. 0 HP / 0 Def Serperior: 184-217 (63.2 - 74.5%) -- guaranteed 2HKO
SpD doesn’t quite OHKO, but does a large amount of damage, and gets rid of your item. It also has U-Turn which does ~50% and lets you switch out to something like Ditto or Calyrex-Shadow that can outspeed it and take it out.
Outspeeds, immune to Dragon, resistant to Grass, OHKOs with behemoth blade. Enough said.
Groundceus’s physical attacking set can OHKO if it has Jolly, as it runs Ice Beam as coverage despite being mostly physical for Zygarde. Physically Defensive also runs ice beam, but serperior is faster. It should OHKO with a specs super effective STAB Leaf Storm…right?
252 SpA Choice Specs Serperior Leaf Storm vs. 252 HP / 0 SpD Arceus-Ground: 378-446 (85.1 - 100.4%) -- 6.3% chance to OHKO
Not always. Usually not, in fact. As for Ice Beam, that OHKOs.
A very strong and common Pokémon that is a staple on just about any team, it has amazing bulk and attack. It resists both of Serperior’s STABs, and it’s bulk is so incredible that…
+4 252 SpA Choice Specs Serperior Draco Meteor vs. 252 HP / 4 SpD Necrozma-Dusk-Mane: 306-360 (76.8 - 90.4%) -- guaranteed 2HKO
It lives a 4+ specs Draco Meteor.
252 SpA Choice Specs Serperior Draco Meteor vs. 80 HP / 0 SpD Necrozma-Dusk-Mane: 102-121 (28.7 - 34%) -- 2.2% chance to 3HKO
DD NDM, even with it’s lack of max HP EVs, shrugs off it’s attacks. So let’s take a look at how NDM fairs on offense.
252+ Atk Necrozma-Dusk-Mane Sunsteel Strike vs. 0 HP / 0 Def Serperior: 216-255 (74.2 - 87.6%) -- guaranteed 2HKO
0 Atk Necrozma-Dusk-Mane Sunsteel Strike vs. 0 HP / 0 Def Serperior: 168-198 (57.7 - 68%) -- guaranteed 2HKO
Pretty great, huh?
Ditto: takes advantage of boosts, destroys with Draco Meteor
Fairyceus: takes a hit, destroys with Judgement.
Mega Rayquaza: is OHKOd by Draco Meteor if Serperior holds Specs or a Life Orb, but outspeeds and OHKOs if the Rayquaza is Jolly or holds a Choice Scarf.
Eternatus: Offensive sets outspeed and annihilate, defensive sets can take a Draco Meteor and strike back with an OHKO.
(Do note that many of the above calculations give Serperior Choice Specs to show it’s max firepower, but this leaves it vulnerable to fairy types or one of the many things that resists grass, and if you don’t hold specs, it’s firepower is even more depressing)
Serperior also doesn’t have much coverage at all. It’s only notable coverage move is Hidden Power (which is weaker than STABs in nearly ever case)
In conclusion, Serperior lacks the bulk, firepower, coverage, and sometimes even the speed to compete with AG’s top Pokémon.