The team already has a strong breaker, revenge killer and a strong defensive core. I find that balance teams like these appreciate having extra bulky Pokemon that can help exert pressure on the opposing team. I wanted something that wouldn't be passive, but would also act as a defensive pivot and would help take the pressure off of Swampert in certain matchups. A Water resist would go a long way here, to prevent water types from running through this team fast. Barraskewda, Inteleon and Walking Wake are all massive threats to the team with no real answer in game, so covering those 3 in our last slot was essential. Obviously something with some kind of hazard would also be beneficial, with all 5 pokemon on the team having a pivoting move something to punish switchs would be much appreciated. Additionally, as the current gameplan into Chien-Pao is pray we never get flinched (Or worse, Lashed out), Stealth Rock makes Corviknight a much more appealing answer. Electrode-Hisui is a big threat, and MGLO Gouging Fire clicks Flare Blitz on Mandibuzz until it burns and then it wins. In a similar vein, Mandibuzz (and by extension the rest of the team) struggles to switch in to Desolate Land Ceruledge. If it's running Solar Blade then Swampert will get OHKOed. Desolate Land Iron Moth running grass coverage in Energy ball or Solar Beam is also a big threat. That's a lot to cover in one slot for certain, but I feel that Water Absorb
does the most good in the final slot here. This is the set I propose:
Diancie @ Leftovers
Ability: Water Absorb
Tera Type: Rock
EVs: 252 HP / 20 Atk / 216 Def / 20 Spe
Impish Nature
- Stealth Rock
- Diamond Storm
- Body Press
- Protect
Stealth Rock gives us our entry hazards to make the most use of all of our U-Turn (or Flip Turn, dw Swampert we get it) Pokemon to make progress in most matchups we will come across. Diamond Storm is our strong Rock Stab that will help us fend off those annoying Desolate Land Fire types, but also acts as a boosting option to help prevent us from being too passive. Body Press is good coverage with Diamond Storm and helps us make use of our defense boosts to put some pressure on opposing removal, particularly opposing Corviknight and Mandibuzz, who can't safely Roost. Protect rounds out the set, giving us a great scouting tool and reliable recovery along with our Leftovers. The EV's are max HP to maximize Leftovers recovery, along with a significant Defense investment. 216 with Impish nature lets us hit a strong defensive benchmark and ensures our Body Press will be doing a lot, especially after a Diamond Storm Boost. 20 Speed EV's let us outspeed a negative nature Goodra-H and threaten it with Body Press (This helps Swampert keep its Assault Vest). 20 EVs are put into Attack to increase the power of Diamond Storm, giving it better rolls into Zapdos, Gouging Fire, Bulky Ceruledge and Heatran.
Some things to note about Diancie filling out the last spot of our team are:
1. We have to play very aggresively into matchups against Electrode-Hisui + a pivot slower than one of our own. Azelf of course can force it out, but beating it will have to come down to burning Chloroblasts PP with Protect and switching between Corviknight and Swampert. Identifying if it's okay to sacrifice one of our flying types will hopefully allow us to burn significant PP and allow for Swampert to hard wall it. If it's running Energy Ball as well as Chloroblast, it's very difficult but getting the damage on it with a U-Turn with a flying type will allow Diamond Storm to kill, if you can get that far. Ultimately the team is quite weak to Electrode-Hisui and Diancie didn't solve that issue. SAD!
2. Diancie completely patchs up our weakness to water types! The only Water types that is a significant threat to Diancie are Tail Glow Manaphy running Energy Ball and Ogerpon-Wellspring, but luckily Unaware Mandibuzz and Intimidate Corviknight should be able to take those on for us. Barraskewda, Inteleon, Latios, Swampert and Latios could freely click Flip Turn against us before, but now we have a significant way to punish them. Swampert really enjoys the support Diancie provides, allowing it to not have to take the full load for handling all of these pokemon. Swampert can also be a little more relaxed around offensive Zapdos, especially after getting hit by a Knock Off, as Diancie takes strong Hurricanes with ease.
3. Desolate Land fire types aren't as big of a threat. Swampert is rejoicing again, as Diancie helps share the defensive load of taking on the tier's strong fire types. Diancie sits on MGLO Gouging Fire lacking Earthquake forever, making Mandibuzz' job easier into that matchup. Diancie can take a Life Orb Flare Blitz into an Earthquake, but we can scout with Protect and hopefully do well with the threat of Mandibuzz in the back. Flare Blitz EQ DD recovery could be scary, but Swampert can help us out against that one. Bulkier Gouging Fire's typically aren't as scary as Diamond Storm will 2HKO all but the most physically invested ones. Diancie can also stomach a Life Orb Bitter Blade into Solar Blade from Desolate Land Ceruledge, working well in tandem with Mandibuzz once again. You can Protect to scout and gain HP, and potentially even pivot to Mandibuzz on a Solar Blade and threaten to OHKO with Foul Play. Diamond Storm is also always killing Ceruledge after Life Orb recoil. Finally Diancie can also take a Fiery Dance into 1.5x Energy Ball from Desolate Land Iron Moth, given Protect's additional 6% recovery. A Fiery Dance boost into Solar Beam will be fatal though, so be aware of the threat that poses. Diamond Storm always kills Iron Moth, so preventing it from getting the boost by refusing to let it come in on our Corviknight is key.
4. Great Tusk will be able to spin on our setter, but with Regenerator not being as common we can hopefully wear them down with Corviknight's Brave Bird, Mandibuzz's Toxic or a strong attack from one of our two choiced breakers. Keep in mind that a Great Tusk also has to be careful of a STAB Moonblast, so they aren't likely to hard switch into Diancie at first, allowing you to play with SR up until your full set is revealed.
5. Chien Pao is much easier to deal with provided we can keep our Stealth Rock up. If our Corviknight gets flinched now we can feel a little more secure in that Diancie will always avoid the 3HKO from -1 Icicle Crash and threaten an OHKO back. Since we have so many pivots we can hopefully heal up our Corv in the mean time or can use it as a sac to allow Diancie to check it once again.
Below are some calcs showing the issues the team faces, and how Diancie addresses them.
252 SpA Choice Specs Inteleon Hydro Pump vs. 248 HP / 252+ SpD Assault Vest Swampert in Heavy Rain: 204-240 (50.6 - 59.5%) -- guaranteed 2HKO
-1 252 Atk Choice Band Barraskewda Flip Turn vs. 252 HP / 136+ Def Corviknight in Heavy Rain: 109-130 (27.2 - 32.5%) -- 63.6% chance to 4HKO after Leftovers recovery
252 SpA Choice Specs Dragon's Maw Walking Wake Draco Meteor over 2 turns vs. 252 HP / 252+ SpD Assault Vest Swampert: 358-424 (88.6 - 104.9%) -- 31.3% chance to 2HKO
252+ Atk Life Orb Gouging Fire Flare Blitz vs. 252 HP / 252+ Def Mandibuzz: 179-212 (42.2 - 50%) -- 0.4% chance to 2HKO
252+ Atk Life Orb Ceruledge Bitter Blade vs. 248 HP / 252+ Def Mandibuzz in Harsh Sunshine: 214-253 (50.5 - 59.8%) -- guaranteed 2HKO
252 SpA Zapdos Hurricane vs. 248 HP / 252+ SpD (Knocked Off) Swampert: 135-160 (33.4 - 39.7%) -- guaranteed 3HKO
-1 252 Atk Choice Band Sword of Ruin Chien-Pao Icicle Crash vs. 252 HP / 216+ Def Diancie: 97-115 (31.9 - 37.8%) -- 0.3% chance to 3HKO after Leftovers recovery
252 SpA Iron Moth Fiery Dance vs. 252 HP / 0 SpD Diancie in Harsh Sunshine: 72-86 (23.6 - 28.2%) -- possible 5HKO after Leftovers recovery
252 SpA Iron Moth Energy Ball vs. 252 HP / 0 SpD Diancie: 146-174 (48 - 57.2%) -- 40.2% chance to 2HKO after Leftovers recovery
252+ Atk Life Orb Ceruledge Bitter Blade vs. 252 HP / 216+ Def Diancie in Harsh Sunshine: 84-100 (27.6 - 32.8%) -- 74.7% chance to 4HKO after Leftovers recovery
252+ Atk Life Orb Ceruledge Poltergeist vs. 252 HP / 216+ Def Diancie: 138-164 (45.3 - 53.9%) -- 2% chance to 2HKO after Leftovers recovery
252+ Atk Life Orb Ceruledge Solar Blade vs. 252 HP / 216+ Def Diancie: 208-247 (68.4 - 81.2%) -- guaranteed 2HKO after Leftovers recovery
40 Atk Diancie Diamond Storm vs. 0 HP / 0 Def Heatran: 108-127 (33.4 - 39.3%) -- guaranteed 3HKO
40 Atk Diancie Diamond Storm vs. 0 HP / 0 Def Zapdos: 258-306 (80.3 - 95.3%) -- guaranteed OHKO after Stealth Rock
40 Atk Diancie Diamond Storm vs. 0 HP / 0 Def Ceruledge: 270-320 (92.7 - 109.9%) -- 56.3% chance to OHKO
40 Atk Diancie Diamond Storm vs. 0 HP / 0 Def Iron Moth: 338-402 (112.2 - 133.5%) -- guaranteed OHKO
216+ Def Diancie Body Press vs. 252 HP / 0 Def Goodra-Hisui: 210-248 (57.6 - 68.1%) -- guaranteed 2HKO
+2 216+ Def Diancie Body Press vs. 248 HP / 244+ Def Mandibuzz: 147-173 (34.7 - 40.8%) -- guaranteed 3HKO (would do double on a Roost)
+2 216+ Def Diancie Body Press vs. 252 HP / 252+ Def Corviknight: 146-172 (36.5 - 43%) -- 97.3% chance to 3HKO after Leftovers recovery
@ Choice Band 
as more as a utility mon rather than a sd set because there are no hazard setters, completes a fwg core and not much utility apart from 
deals with that pretty well. 
that resists either or both dragon and fairy allowing for more flexible plays. Lastly, it can set hazards up and can be spinblocked by 
