Presenting...
A lost heros and Uselesscrab joint production...
Yes I know that Keldeo-Resolute only exists when it has Secret Sword. Isn't this ironic???
A lost heros and Uselesscrab joint production...

Keldeo @ Waterium Z
Ability: Justified
EVs: 252 HP / 80 Def / 116 SpA / 20 SpD / 40 Spe
Modest Nature
IVs: 0 Atk
- Hydro Pump
- Secret Sword
- Calm Mind
- Icy Wind
Ability: Justified
EVs: 252 HP / 80 Def / 116 SpA / 20 SpD / 40 Spe
Modest Nature
IVs: 0 Atk
- Hydro Pump
- Secret Sword
- Calm Mind
- Icy Wind

Keldeo @ Fightinium Z
Ability: Justified
EVs: 252 HP / 80 Def / 116 SpA / 20 SpD / 40 Spe
Modest Nature
IVs: 0 Atk
- Focus Blast
- Hidden Power [Rock]
- Calm Mind / Surf
- Icy Wind
Ability: Justified
EVs: 252 HP / 80 Def / 116 SpA / 20 SpD / 40 Spe
Modest Nature
IVs: 0 Atk
- Focus Blast
- Hidden Power [Rock]
- Calm Mind / Surf
- Icy Wind
Yes I know that Keldeo-Resolute only exists when it has Secret Sword. Isn't this ironic???
There's a lot to explain with this pair of Keldeo 1v1 sets, so I'll start a little bit of backstory. I started toying around with this Pokemon last generation. Keldeo's solid overall bulk allows it to live some unexpected hits, but it needs a slight power boost to OHKO from low HP once it's lived them. Custap Berry, which would allow me to get off multiple attacks, soon became unreleased, and it, like Petaya Berry, proved too unreliable an item choice, as many hits left me outside the 25% range.
Fortunately, Generation VII brought with it the mindless power boost of Z-moves, and so I was able to make use of the first set above. The central strategy is to Icy Wind + Z-move vs. faster attackers and Sturdy Pokemon. lost heros is to thank (or to blame) for a lot of the EVs. The Defense investment, which is specifically tailored for a +2 Mega Gyarados Outrage, also allows it to survive hits a variety of common physical attackers — Crustle, Donphan, Kyurem-Black, Sawk, etc. The Special Defense is for Mega Charizard Y's Solar Beam and also allows. Finally, the Speed outspeeds all non-Jolly Mega Gyarados (the vast majority). Here's an assortment of calcs that are relevant for both sets:
+2 252+ Atk Mold Breaker Gyarados-Mega Outrage vs. 252 HP / 80 Def Keldeo: 327-385 (84.7 - 99.7%) -- guaranteed 2HKOFortunately, Generation VII brought with it the mindless power boost of Z-moves, and so I was able to make use of the first set above. The central strategy is to Icy Wind + Z-move vs. faster attackers and Sturdy Pokemon. lost heros is to thank (or to blame) for a lot of the EVs. The Defense investment, which is specifically tailored for a +2 Mega Gyarados Outrage, also allows it to survive hits a variety of common physical attackers — Crustle, Donphan, Kyurem-Black, Sawk, etc. The Special Defense is for Mega Charizard Y's Solar Beam and also allows. Finally, the Speed outspeeds all non-Jolly Mega Gyarados (the vast majority). Here's an assortment of calcs that are relevant for both sets:
+2 252+ Atk Crustle Continental Crush (200 BP) vs. 252 HP / 80 Def Keldeo: 308-363 (79.7 - 94%) -- guaranteed 2HKO
252+ Atk Donphan Tectonic Rage (180 BP) vs. 252 HP / 80 Def Keldeo: 306-360 (79.2 - 93.2%) -- guaranteed 2HKO
252+ Atk Donphan Ice Shard vs. 252 HP / 80 Def Keldeo: 22-27 (5.6 - 6.9%) -- possibly the worst move ever
252+ Atk Teravolt Kyurem-Black Fusion Bolt vs. 252 HP / 80 Def Keldeo: 294-346 (76.1 - 89.6%) -- guaranteed 2HKO
252+ Atk Sawk All-Out Pummeling (190 BP) vs. 252 HP / 80 Def Keldeo: 331-391 (85.7 - 101.2%) -- 12.5% chance to OHKO
252 SpA Charizard-Mega-Y Solar Beam vs. 252 HP / 20 SpD Keldeo: 326-384 (84.4 - 99.4%) -- guaranteed 2HKO
+1 252+ SpA Genesect Thunderbolt vs. 252 HP / 20 SpD Keldeo: 326-384 (84.4 - 99.4%) -- guaranteed 2HKO
One matchup that's really confusing is the Mega Gyarados one, but it's generally favorable on the whole. Gyarados usually tries to Dragon Dance, so Icy Winding thrice and then using a Z-move is the best bet. Still, don't make my mistake. Actually bring a second method of beating Gyarados.
The central difference between the two sets can be summarized as follows: Waterium Z is better against the entirety of the ladder, but has difficulty with certain threats (e.g. Scarf Kyurem-B). Fightinium Z is incredibly good at handling the specific Pokemon the set is designed to beat (Kyurem-B, both Charizards (it uses HP Rock for Y, unlike the Water set), Crustle, Donphan, and Mega Gyarados), but struggles against either stall (if it opts for Surf) or Pokemon it would normally counter as a Water-type (if it opts for Calm Mind).
This was far from my clearest post ever, and the general strategy is pretty confusing, so here are some replays that I think explain it far better:
Either Z-move vs. Donphan
Charizard Y
Ideal Gyarados outcome
Good Icy Wind opportunity
Calm Mind vs. Ghostium Z Aegislash
bonus: Weird roll of the day vs. Magnezone