I am also of the opinion that this episode is purely just to show Darkrai off battling while Ash watches in the stands because of how the tile says "Darkrai appears!" instead of Ash or one of his Pokemon Vs Darkrai
I'd imagine either we see Ash battling some of his Semi Final in this probably in the first half and then Takuto's one in the second half or the next DP190 is about Ash's Semi-Final battle but it's unlikely but not very unlikely just unlikely lol
If this episode isn't Ash vs Takuto(Which I hope is the case), then I see/hope it plays out like this:
-Ash battles his semifinal opponent, showing the six remaining Pokemon that haven't battled yet(Muk, Bayleaf, Totodile, Corphish, Torkoal, and Glalie).
An episode is around 22 mins long(Without the ending and preview for next episode), take off two mins for the recap and opening and there is about 20mins of possible battle time. Takuto's match doesn't need to be over 5 mins long, and I would say it should be 3 mins at the most. Giving Ash at less 15 mins(If Takuto's match does take 5 mins) to 17 mins of battle time. There probably will be some talking and other non battling things, which would take no more then 5 mins off. So at less 10-12 mins of battle time for Ash. I'll break the time down in the battles now:
Ash vs Random person:
Corphish vs Yanmega-It learned Metal Claw but faints. 40 seconds
Torkoal vs Yanmega-Yanmega is recalled. 5 seconds
Torkoal vs Walrein-It learned Sludge Bomb but faints. 35 seconds
Glalie vs Walrein-Glalie learned Sheer Cold and/or Shadow Ball, and wins. 50 seconds
Glalie vs Camerupt-Glalie is recalled. 5 seconds
Totodile vs Camerupt-Totodile learned Hydro Pump and Aqua Tail and wins. 25 seconds
Totodile vs Yanmega-Totodile evolves right away and learned Super Power after that, it also learned Ice Fang before evolving which defeated Yanmega, Croconaw is recalled. 1 minute 35 seconds
Bayleaf vs Espeon -Bayleaf learned Solarbeam, Magical Leaf, and either Aromatherapy or Toxic. Bayleaf wins. 1 minute 45 seconds
Bayleaf vs Magnezone-Bayleaf faints. 1 minute
Glalie vs Magnezone-Glalie faints. 50 seconds
Croconaw vs Magnezone-Croconaw wins and is recalled. 2 minutes
Muk vs Carnivine-Muk learned Gunk Shot and Hyper Beam and wins with Muk and Croconaw still kicking. 3 minutes
Total amount of time is around 13 minutes.(I don't know if People would consider it rushed or whatever, I'm sure it would look better then it sounds).
-Then the next match between Takuto and a random trainer begins. Takuto's Darkrai is the only Pokemon he uses and wins without taking a hit(and Knocks out all the Pokemon with one hit).
-Episode ends with Ash worrying about his battle with Takuto.
I think they can do this so long as they make it purely a battling episode without anything else(Like Ash training his Pokemon as seen in DP 185).