Overall, I'd say Mienshao is worth using in Gen 7. It has an amazing moveset that allows it to weaken walls (or most Pokémon it general) with Knock Off, can be a Physically-Attacking Pivot with U-Turn, and can get some easy damage first turn with Fake Out. It does have some weaknesses though, which I will cover after the set. Here is the moveset provided by Smogon, and what I believe to be one of the best movesets for the Regenerator Ability:
Mienshao @ Assault Vest
Ability: Regenerator
EVs: 100 HP / 156 SpD / 252 Spe
Jolly Nature / Impish Nature (what you have)
- High Jump Kick (Lv. 36)
- Knock Off (12 Battle Points)
- U-turn (Lv. 41)
- Fake Out (Lv. 13)
With this moveset, Mienshao can take hits decently well, especially on the Special side, thanks to its EVs and Assault Vest. However, the Assualt Vest is only obtained in USUM by getting 48 points in the Battle Tree, so if you don't want to go through that, you could get a Focus Sash (Found In Poni Wilds), or Leftovers (Found via Pickup). The Assault Vest helps deal with threats such as Hydreigon and Lati@s. High Jump Kick is an overall powerful move that can OHKO many Pokémon weak to it, like Empoleon and Hydreigon. Knock Off can remove the items of a Pokémon. This helpful against wall like Amoongus, or really any Pokémon with an item that improves their power or longevity. It also is coverage against Ghost or Psychic Types, but if they are Physically Bulky, I'd consider switching out with U-Turn. Speaking of U-Turn, it is helpful to for overall pivoting. As an additional bonus, it gets health back from regenerator. Finally fake out is used to get some easy damage on its first turn it can battle. This can disrupt Pokémon from using certain moves, like Toxic or Calm Mind, that would help achieve their goal. However, this can be replaced by a move like Stone Edge or Poison Jab for more Coverage.
However, your Mienshao has the Impish Ability. While it isn't bad, and can make it take Physical Attacks better, Jolly is usually preferred to give it a speed boost that helps it pull of moves first in a turn. This is especially important for moves like U-Turn.
So overall, I would use the Mienshao, however it's Nature may hold it back some. Unfortunately, there are no mints USUM, meaning you must stick with it. So like I said, it may not be as good as Jolly, but it could definitely be helpful in battle, as it increases it's longevity slightly.
EDIT: I Forgot that you might want a more in-game moveset as well, so here is that:
Mienshao @ Any Listed Above, along with Choice Scarf, Life Orb, or a Z Crystal, like Fightium Z
Abiltiy: Regenerator
EVs: 252 Atk, 252 Spe, 4 SpD
Impish Nature
- High Jump Kick / Drain Punch
- U-Turn / Poison Jab / Stone Edge
- Knock Off
- Fake Out / Poison Jab / Stone Edge
This moveset is somewhat similar. The EVs (which aren't a necessity in-game) can be used to deal as much damage as possible, as quickly as possible. Drain Punch can be used to heal a fair amount while dealing damage. Poison is coverage against Fairy Types, while Stone Edge is for Flying Types (I'd recommend switching if the Fairy or Flying Types being faced are very fast). Stone Edge can also be replaced with Rockslide for better accuracy, along with it being able to hit multiple opponents in the occasional Double Battle.
I Hope I Helped (and congrats on the shiny)!
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