I know it's a little off kilter, but;
Moveset Submission:
Name: Baited (Shell) Trap
Move 1: Shell Trap
Move 2: Spiky Shield
Move 3: Sucker Punch / Ice Shard
Move 4: Hidden Power Grass / Hidden Power Fire / Hidden Power Ice
Ability: Poison Heal
Item: Toxic Orb / Kee Berry
EVs: 252 Def / 252 SpA /
Nature: Quiet
This does have the issue of slightly messing with Ferrothorn - but Hidden Power Fire already put paid to Ferrothorn with Modest+Full Investment outdamaging it as a Ferrothorn "lure" if you wanted it to. Jumbao is the only other listed counter who'd be badly affected by the Fire type move, especially if it had Drought, but Jumbao doesn't use Phys anyway.
252+ SpA Araquanid Hidden Power Fire vs. 252 HP / 56 SpD Ferrothorn: 176-208 (50 - 59%) -- 67.2% chance to 2HKO after Leftovers recovery
252+ SpA Araquanid Hidden Power Fire vs. 252 HP / 168 SpD Ferrothorn: 160-192 (45.4 - 54.5%) -- 5.1% chance to 2HKO after Leftovers recovery
32 Atk Ferrothorn Power Whip vs. 0 HP / 0 Def Araquanid: 118-139 (36.5 - 43%) -- 6.5% chance to 3HKO after Poison Heal
With our Speed so low, there's no way we're taking out Necturna in normal play, so we either need Trick Room, or we need Priority - the only real priority available to make a dent is one of Ice Shard, or Sucker Punch - no Aqua Jet for obvious reasons. Combining Spiky Shield with Sucker Punch gives some real clout to put a dent in offensive Necturna, especially if it has already taken chip damage while throwing in some additional. If Necturna has Stone Edge, it's good night gracie, there's not much we can do about that. However, it can still be made to pay with it. To prevent Necturna trying to Phantom Force to bypass Spiky Shield, CAP25w's Shell Trap requires it to be at +4 to nab the kill, while the high 2HKO Chance with Ice Shard after Rocks puts a big dent on Necturna's parade, especially if it's switching in with help from a Rough Skin/Rocky Helmet support elsewhere in the team. HP Ice is a 2HKO after rocks as an option.
4 Atk Araquanid Sucker Punch vs. -1 0 HP / 0 Def Necturna: 146-172 (54.2 - 63.9%) -- guaranteed 2HKO
4 Atk Araquanid Ice Shard vs. -1 0 HP / 0 Def Necturna: 84-100 (31.2 - 37.1%) -- 79.7% chance to 3HKO
4 Atk Araquanid Ice Shard vs. -2 0 HP / 0 Def Necturna: 112-132 (41.6 - 49%) -- 80.1% chance to 2HKO after Stealth Rock
252+ SpA Araquanid Shell Trap vs. -1 0 HP / 4 SpD Necturna: 332-392 (123.4 - 145.7%) -- guaranteed OHKO
+2 252+ Atk Necturna Power Whip vs. 0 HP / 252 Def Araquanid: 301-355 (93.1 - 109.9%) -- 56.3% chance to OHKO after Poison Heal
+2 252+ Atk Necturna Phantom Force vs. 0 HP / 252 Def Araquanid: 226-267 (69.9 - 82.6%) -- guaranteed 2HKO after Poison Heal
252+ Atk Necturna Never-Ending Nightmare (175 BP) vs. 0 HP / 252 Def Araquanid: 220-259 (68.1 - 80.1%) -- guaranteed 2HKO after Poison Heal
+2 252+ Atk Necturna Stone Edge vs. 0 HP / 252 Def Araquanid: 334-394 (103.4 - 121.9%) -- guaranteed OHKO
Spiky Shield in the face of the Power Whip forces it to go for Phantom Edge, which risks the OHKO Shell Trap after being set up, while if it's at 0, it can't make a dent.
Against Mega-Scizor;
252+ SpA Araquanid Shell Trap vs. 248 HP / 0 SpD Scizor-Mega: 520-612 (151.6 - 178.4%) -- guaranteed OHKO
+2 0+ Atk Scizor-Mega Knock Off (97.5 BP) vs. 0 HP / 252 Def Araquanid: 161-190 (49.8 - 58.8%) -- 17.6% chance to 2HKO after Poison Heal
It suffers against Special Sets unless Hidden Power Fire is ran as a special option to deal with Syclant, which is an option if you don't need Grass to take on Mega-Swampert, or Ice to threaten Zygarde and Fidgit. Shell Trap is irrelevant to special sets (the majority of which Syclant uses) - Hidden Power Fire would kill it with some SpA investment regardless. However, Phys sets can't 2HKO at +2 without Life Orb, Spiky Shield breaks the sash, provides healing, and gets gibbed in return by the Shell Trap.
252+ SpA Araquanid Hidden Power Fire vs. 0 HP / 0- SpD Syclant: 336-396 (119.5 - 140.9%) -- guaranteed OHKO
252 SpA Syclant Bug Buzz vs. 0 HP / 0 SpD Araquanid: 138-163 (42.7 - 50.4%) -- guaranteed 3HKO after Poison Heal
+3 252 SpA Syclant Bug Buzz vs. 0 HP / 0 SpD Araquanid: 343-405 (106.1 - 125.3%) -- guaranteed OHKO
+2 0 Atk Syclant Leech Life vs. 0 HP / 252 Def Araquanid: 144-171 (44.5 - 52.9%) -- guaranteed 3HKO after Poison Heal
Zygarde isn't making much headway even with Outrage if Poison Heal is active and Hidden Power Ice was selected. However, any Substitute Sets are still rather painful to chew through.
252+ SpA Araquanid Hidden Power Ice vs. 4 HP / 0 SpD Zygarde: 220-260 (61.4 - 72.6%) -- guaranteed 2HKO after Stealth Rock.
252+ Atk Zygarde Outrage vs. 0 HP / 252 Def Araquanid: 133-157 (41.1 - 48.6%) -- 99.8% chance to 3HKO after Poison Heal
252+ Atk Zygarde Devastating Drake (190 BP) vs. 0 HP / 252 Def Araquanid: 211-249 (65.3 - 77%) -- guaranteed 2HKO after Poison Heal
Colossoil's Sucker Punch can be a pain - and it's pretty fat. Facade still allows a kill, but it's a bit lairy when you involve Burn chip, Spiky Shield, and 25w's own fatness giving it maybe only a 25% 3HKO chance vs Shell Traps 2HKO after Burn and Chip, if my maths is correct. (-25% from Spiky Shield, -12.5% from Burn Chip = -37.5%, healed 37.5% from Poison Heal)
252+ SpA Araquanid Shell Trap vs. 0 HP / 0 SpD Colossoil: 170-200 (41.7 - 49.1%) -- 82.4% chance to 2HKO
252+ Atk Guts Colossoil Facade (140 BP) vs. 0 HP / 252 Def Araquanid: 178-210 (55.1 - 65%) -- 96.1% chance to 2HKO after Poison Heal
Mega Swampert; Hidden Power Grass or nah. It's a rain mon, fire is worthless. You can however stall out its best days in the Rain by just shoving your fat ass down its throat and dealing a ton of chip. Works wonders with dedicated support to remove the other setters like Tomo and Pelipper. Spiky Shield and HP Ice can not only stall out the rain, but can also kill quicker than Rain Boosted Waterfall can thanks to Poison Heal.
252+ SpA Araquanid Hidden Power Grass vs. 0 HP / 0 SpD Swampert-Mega: 192-228 (56.3 - 66.8%) -- guaranteed 2HKO
252+ SpA Araquanid Hidden Power Ice vs. 0 HP / 0 SpD Swampert-Mega: 48-57 (14 - 16.7%) -- possible 6HKO
252+ Atk Swampert-Mega Earthquake vs. 0 HP / 252 Def Araquanid: 74-87 (22.9 - 26.9%) -- possible 7HKO after Poison Heal
252+ Atk Swampert-Mega Waterfall vs. 0 HP / 252 Def Araquanid in Rain: 89-105 (27.5 - 32.5%) -- possible 5HKO after Poison Heal
Fidgit's Earth Power is outdamaged by HP Ice while Poison Heal recovers almost all damage.
If Choice Locked into Secret Sword, Keldeo is Shell Trap meat, but Hidden Power Grass combined with Spiky Shield's safe healing lets you whittle your way through his HP while Scarfed variants struggle to damage you even with STAB or SE physical ones with Stone Edge getting off lightly from Spiky Shield.
252+ SpA Araquanid Hidden Power Grass vs. 0 HP / 4 SpD Keldeo: 112-134 (34.6 - 41.4%) -- guaranteed 3HKO
252 SpA Choice Specs Keldeo Hydro Pump vs. 0 HP / 0 SpD Araquanid: 137-162 (42.4 - 50.1%) -- guaranteed 3HKO after Poison Heal
There isn't much Arghonaut is doing to either 25w or us to it; however, it rarely runs Gunk Shot, although I can see it doing it a fair bit now, although wanting Recover, Spikes, Circle Throw, Gunk Shot and now Aqua Jet with 25f making a showing, I'm going to hazard a guess that Aqua Jet will be more popular to OHKO 25f, rather than potentially 5HKOing 25w should it never use the Spiky Shield.
252+ SpA Araquanid Hidden Power Grass vs. 252 HP / 4 SpD Arghonaut: 104-124 (25.1 - 29.9%) -- 0.1% chance to 4HKO after Leftovers recovery
Jirachi can't actually touch us without most of the damage being healed off - it's usually physical, but has little to no investment, instead preferring to Flinch Hax. 25w's low speed means that the -3 priority isn't an issue more than usual, and if the Flinch Hax fails, it's a 2HKO after leftovers.
0 Atk Jirachi Iron Head vs. 0 HP / 252 Def Araquanid: 32-38 (9.9 - 11.7%) -- possibly the worst move ever
252+ SpA Araquanid Shell Trap vs. 248 HP / 196 SpD Jirachi: 214-254 (53.1 - 63%) -- guaranteed 2HKO after Leftovers recovery
However, we do have one sticking point in the switch-ins - Mega Gyarados. With high special defense, and only HP Grass, you're better off relying on something like 25g to kill it with Electro-Facade or something.
96+ Atk Mold Breaker Gyarados-Mega Crunch vs. 0 HP / 252 Def Araquanid: 109-130 (33.7 - 40.2%) -- 99.9% chance to 4HKO after Poison Heal
Hawlucha gets pressured pretty hard - if it's taken some chip damage, a Shell Trap, Ice Shard and Spiky Shield deals around 90% damage to it at +1, possibly even able to nab a KO at full HP if it's not got the Seed.
252+ SpA Araquanid Shell Trap vs. 96 HP / 36 SpD Hawlucha: 179-211 (55.7 - 65.7%) -- guaranteed 2HKO
4 Atk Araquanid Ice Shard vs. +1 96 HP / 0 Def Hawlucha: 48-58 (14.9 - 18%) -- possible 6HKO
Landorus-T
224 Atk Landorus Stone Edge vs. 0 HP / 252 Def Araquanid: 156-184 (48.2 - 56.9%) -- 2.3% chance to 2HKO after Poison Heal
4 Atk Araquanid Ice Shard vs. 32 HP / 4 Def Landorus: 120-144 (36.6 - 44%) -- guaranteed 3HKO
252+ SpA Araquanid Hidden Power Ice vs. 32 HP / 0- SpD Landorus: 280-332 (85.6 - 101.5%) -- 12.5% chance to OHKO
Spiky Shield allows the guarantee you won't be 2HKO'd if you switch into a defog, but unless you've got HP Ice AND Ice Shard, you're going to be struggling with LandoT. If it switches into HP Ice, it will still likely be OHKO'd by Ice Shard after.
Mega-Crucibelle cannot be touched - its high special defense means that unless the low accuracy of Head Smash means it misses 2 in a row, even replacing Hidden Power with Hydro Pump will do little. Versus Tapu Bulu, if it's SD'd up, it's capable of OHKO with Horn Leech - Ice Shard can grab a kill however if it was super low. Tomo, Koko, Krilowatt, Jumbao (might need to watch out for HP Fire + Ice Shard if it at 50%)
Pajantom is a 3HKO with Spirit Shackle/Brave Bird if there's no poison heal, but that turns it into a 4HKO, allowing HP Ice and/or Ice Shard to have some fun.
You get the picture. Any more calcs requested, let me know.
Changes to the Threatlist;
Mega-Gyarados; becomes a Check and Counter
Mega-Crucibelle; becomes a Check and Counter
Ferrothorn; becomes a Switch In