Thinking ahead towards the Skeledirge raid, I think we can probably expect a moveset consisting of Torch Song, Alluring Voice, Hex and Will-O-Wisp. Shadow Ball and Scorching Sands are also possible.
For scripted moves, I would say turn 1 Flame Charge could be a thing. Sunny Day and Heat Wave also likely choices.
Unaware and constant SpA boosts from Torch Song are going to be a major threat, and they will likely be the biggest hurdle we need to overcome with our strategies.
With these in mind, we might consider following counters:
Manaphy @ Shell Bell / Covert Cloak
Ability: Hydration
Tera Type: Water
EVs: 4 HP / 252 SpA / 252 SpD
Modest Nature
- Skill Swap
- Surf
- Tail Glow
- Rain Dance
With Skill Swap we can solve the issue of both Unaware and Torch Song in one move. From there, Tail Glow into Surf will be immensely powerful. One major thing to consider is that Alluring Voice can confuse you on the turns you go for Tail Glow which is why Covert Cloak can be considered as an option. Take Heart is an alternative setup move in case you'd prefer a slower and bulkier approach.
Diancie @ Shell Bell
Ability: Clear Body
Tera Type: Rock
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Skill Swap
- Calm Mind
- Power Gem
Diancie can also pull off similar tricks as Manaphy with Skill Swap and Calm Mind. Diancie being naturally slower than Skeledirge can be a benefit in avoiding the confusion from Alluring Voice.
Blissey @ Shell Bell
Ability: Natural Cure
Tera Type: Psychic
EVs: 4 HP / 252 SpA / 252 SpD
Modest Nature
- Skill Swap
- Calm Mind
- Stored Power
If you want to use a Pokémon that never dies, Blissey can be your choice. Even with Tera Psychic granting you weakness to Ghost, Blissey's special bulk should be more than enough to handle it. And Stored Power should be strong enough at +6 in both of your special stats.
It's worth noting that several Pokémon get access to Skill Swap and Calm Mind, and can use either Stored Power or Tera Water Tera Blast for offense. These options include

,

,

,

and many more, so pick whichever feels right. I think these options should suffice as decent counters against almost anything Skeledirge could pull off.
Also, Soundproof Kommo-o will probably hard wall Skeledirge, although you might want to watch out for Will-O-Wisp + Hex combo.