There are plenty of options to counter stall in Nat Dex AG. Which one to use, obviously depends on your team. Some of the easiest to use and to fit in the team include:
PerishPool Arceus
Arceus-Water @ Splash Plate
Ability: Multitype
EVs: 248 HP / 128 Def / 132 Spe
Bold Nature
IVs: 0 Atk
- Earth Power / Ice Beam / Toxic
- Recover
- Whirlpool
- Perish Song
Arceus-Dark [Arceus Fairy]@ Dread Plate / Pixie Plate
Ability: Multitype
EVs: 248 HP / 132 SpD / 128 Spe
Calm Nature
IVs: 0 Atk
- Judgement
- Recover
- Whirlpool
- Perish Song
Perish Song + Whirlpool is really scary aganist stall teams. This is because most stall teams rely on Chansey or other Arceus formes to switch into a special set of Arceus to scout. Due to their passiveness, they're not usually able to force your Arceus out, meaning you can easily trap with Whirlpool and faint their special tank with PerishPool. Once Chansey / Tyranitar, etc. have been taken down, Yveltal, Calyrex-S and even another CM Arceus can set-up and sweep.
Note that other CM Arceus are also helpful vs stall, you have to deal with Confide Chansey first as it's one of their counters to CM Arceus. Your Arceus wouldn't really mind taking a Toxic in exchange for their Chansey. Note that while it looks like "Chansey can just Teleport", that move is not easy to fix due to the fact that you can't use Confide with Teleport. Without Confide, Chansey is not able to beat CM Arceus or Xerneas properly.
Strong set-up sweepers (Mega Rayquaza, Primal Kyogre, Calyrex-Shadow, etc.)
Rayquaza-Mega @ Life Orb
Ability: Delta Stream
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Swords Dance
- Earthquake / V-create
- Dragon Ascent
- Extreme Speed
Kyogre-Primal @ Blue Orb
Ability: Primordial Sea
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Calm Mind
- Water Spout
- Thunder / Origin Pulse
- Ice Beam
Calyrex-Shadow @ Buginium Z
Ability: As One (Spectrier)
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Nasty Plot
- Astral Barrage
- Pollen Puff
- Baton Pass
Pokemon that have insane raw stats combined with set-up moves can be really annoying. They usually claim a number of kills, or heavily dent them for other team members to sweep through weakened teams. The most notable mentions are obviously Swords Dance Mega-Rayquaza, Calm Mind Primal Kyogre and Savage Spinout Calyrex-Shadow.
If you manage to chip Lugia even at least to 90%, Mega Rayquaza can win the game from there. SD MRay also rips apart bulky Arceus formes thanks to its sheer strength. An alternative is Band MRay, which can still threaten stall without needing any support to chip Lugia. The only thing is, however you're vulnerable to getting Pursuit trapped to Tyranitar if you get locked into Dragon Ascent with the defense drops. However, Tyranitar never switches to you unless revenge killing, which is a good thing. Earthquake OHKOes Tyranitar at +2, though V-create is generally better at breaking Necrozma-DM (which is rare on stall though.)
Calm Mind Primal Kyogre is just another great example vs stall. A +1 SpA Water Spout into unboosted Spout (after Chansey's Confide) after a slight chip is enough to break past Chansey. Remember, Chansey never stays at full which means you're not having much troubles around the pink blob. After Chansey is down, nothing in a stall team can really tank a hit apart from Giratina, which can at max, just Toxic you. Buginium Z Calyrex-S is really scary, as you OHKO Tyranitar and DarkCeus at +1 or force them to Dynamax at least. Baton Passing boosts is also an excellent way if the opposing Chansey is chipped.
+2 252+ Atk Life Orb Rayquaza-Mega Earthquake vs. 252 HP / 4 Def Tyranitar: 730-860 (180.6 - 212.8%) -- guaranteed OHKO
+2 252+ Atk Life Orb Rayquaza-Mega Dragon Ascent vs. 252 HP / 240+ Def Arceus-Ground: 458-539 (103.1 - 121.3%) -- guaranteed OHKO
+1 252+ SpA Kyogre-Primal Water Spout (150 BP) vs. 4 HP / 252+ SpD Eviolite Chansey in Heavy Rain: 360-424 (56 - 66%) -- guaranteed 2HKO
252+ SpA Kyogre-Primal Water Spout (150 BP) vs. 4 HP / 252+ SpD Eviolite Chansey (36% health) in Heavy Rain: 241-285 (37.5 - 44.3%) -- guaranteed OHKO after Stealth Rock
+2 252 SpA Calyrex-Shadow Savage Spin-Out (175 BP) vs. 252 HP / 252+ SpD Tyranitar in Sand: 438-516 (108.4 - 127.7%) -- guaranteed OHKO
+2 252 SpA Calyrex-Shadow Savage Spin-Out (175 BP) vs. 252 HP / 252+ SpD Arceus-Dark: 578-682 (130.1 - 153.6%) -- guaranteed OHKO
Mixed Yveltal
Yveltal @ Life Orb
Ability: Dark Aura
EVs: 4 HP / 252 SpA / 252 Spe
Mild Nature
- Focus Blast
- Oblivion Wing
- Heat Wave
- Knock Off
Mixed Yveltal abuses Chansey with a physical Knock Off, and can beat Tyranitar with a unexpected Focus Blast. This set might be probably the most gimmicky, but it works well vs stall teams, as they rely on Chansey + Tyranitar to wall Yveltal which this set can easily get past.
252+ SpA Life Orb Yveltal Focus Blast vs. 252 HP / 252+ SpD Tyranitar in Sand: 364-432 (90 - 106.9%) -- 43.8% chance to OHKO
4 Atk Life Orb Dark Aura Yveltal Max Darkness vs. 248 HP / 252+ Def Eviolite Chansey: 374-442 (53.2 - 62.8%) -- guaranteed 2HKO
Hope this helps!
Source: Searching around the sets and teams used in tours, also shoutouts to Nevelle for helping me
