I'm a really bad competitive battler, but I want to help you and I have been looking for the answer. I think the best one is astonish.
The Pokemon that can learn the move and handle the Dark and Steel type is Toxicroak, and I don't know if Toxicroak is a good Pokemon for competitive battling but this is the moveset for Toxicroak.
Toxicroak @ Focus Sash
Ability: Poison Touch
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Astonish
- Drain Punch
- Poison Jab
- Toxic/Sucker Punch
So this is the set for Toxicroak, I don't think is bad but like I said, I'm not that good at competitive. Astonish is for the Weakness Policy, Drain Punch covers Dark and Steel types and heals, Poison Jab is a good stab move, and it's up to you if you want Toxic or Sucker Punch, and we want Toxicroak as fast as we can so it could activate the Weakness Policy first. Focus Sash because of its poor defenses.
I hope you like my answer and if you don't like it please accept it because I'm not a good competitive battler. If there's something that needs to be replaced please tell me in the comments because I want to know :)
I have another set if you don't want to use Toxicroak set. It will be shadow ball Gallade. but you need to max out your SpD EVs. Please read the set for Gallade first.
Gallade (M) @ Leftovers
Ability: Steadfast
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
IVs: 0 SpA
- Shadow Ball
- Drain Punch/Close Combat
- Psycho Cut/Zen Headbutt
- Swords Dance
Okay hear me out. Shadow Ball is for the weakness policy but Gallade's Special Attack IVs are 0 (it's important to make Gallade's special attack IVs zero). Drain Punch/Close Combat is for Dark and Steel types. Swords Dance is for set up. Here is the the damage calculator for 0 IVs special attack and -SpA nature gallade using shadow ball against 252 EVs SpD Polteageist.
0- SpA Gallade Shadow Ball vs. 0 HP / 252 SpD Polteageist: 44-52 (16.8 - 19.9%) -- guaranteed 6HKO
Possible damage amounts: (44, 44, 44, 44, 46, 46, 46, 46, 48, 48, 48, 48, 50, 50, 50, 52)
And this is the amount of damage Toxicroak does to 252 Def Polteageist.
**252 Atk Toxicroak Astonish vs. 0 HP / 252 Def Polteageist: 60-72 (22.9 - 27.5%) -- 58.3% chance to 4HKO
Possible damage amounts: (60, 60, 62, 62, 64, 64, 64, 66, 66, 66, 68, 68, 68, 70, 70, 72)
So, the best set to use is Gallade, and Gallade is really strong and I think the set is not that bad, but remember what I said about my competitive skill is not that good:). but remember! -SpA nature(recommend jolly nature) and 0 IVs SpA attack.
hope I helped!
