Skeledirge @ Heavy-Duty Boots
Calm - Unaware
252 HP, 252 Sp. Def, 4 Defense
Tera Type: Fairy / Water
Skeledirge is an interesting case of a Pokemon. Fire/Ghost, while traditionally awkward defensive typing, does give it some key resistances and immunities to work with. An immunity to Fighting, along with resistances to Ghost, Bug, Grass, Fire, and Poison among others, let Skeledirge wall some scary threats like Iron Valiant, Volcarona, Iron Moth, and Breloom. Unfortunately, outside of that select handful of threats... that's about all Skeledirge walls, at least without Terastalyzing. Many of the tier's scariest breakers can threaten Skeledirge with their STABs, such as Chi-Yu, Annihilape, Chien-Pao, Dragapult, Gholdengo, Great Tusk... the list goes on and on.
Terastalyzing can help mitigate some of these problems, turning Skeledirge into a Clefable-at-home or Suicune-at-home, softening blows from the likes of Dragonite, Annihilape, or Dragapult, but many of these breakers will destroy Skeledirge no matter what he Terastalyzes into, such as Chi-Yu. Spreading Will-O-Wisp on switches helps immensely as well, as a threat like Great Tusk or Annihilape, if it lacks Rest, becomes much less scary after a Burn.
Torch Song is a neat little STAB move. 80 base power and guaranteed to raise the user's Sp. Attack. It's also a sound move and thus bypasses Substitute, which can be helpful with things like Shed Tail and Substitute + Nasty Plot Gholdengo running around.
Hex is a pretty solid Ghost STAB, especially against a status-afflicted opponent. Skeledirge pairs fantastically with Toxic Spike setters like Glimmora to help break down bulkier opponents, but needs to be a bit cautious as Toxic Spikes can prevent Skeledirge from Burning an opponent that it needs burnt to wall.
Speaking of burns, Will-O-Wisp comes in to neuter the Attack stat of threatening physical attackers like Great Tusk and Iron Valiant, and in general is just a great move to toss out if you're worried about what your opponent might bring in. A lot of the Pokemon that threaten Skeledirge are physical attackers and thus hate burns.
Slack Off is pretty self-evident. Recovery keeps Skeledirge healthy to wall threats throughout the course of the match. Be somewhat sparing with it as its PP (along with the PP of all other recovery moves that were formerly 10/16) was nerfed to 5/8 in SV, which limits your ability to indefinitely stall opponents the way defensive Pokemon were capable of in previous generations. Running Leftovers over Heavy-Duty Boots can alleviate some pressure from Slack Off if you find yourself running out of PP regularly, but Skeledirge is vulnerable to all forms of hazards and with Gholdengo seeing significant popularity and a lack of reliable defoggers that can beat it, removing hazards is sometimes easier said than done.
EV spread is customizable. Max Sp. Def synergizes well with Will-O-Wisp but if you have a dedicated special wall like Blissey, investing more heavily into physical Defense to take physical moves before Will-O-Wisp lands is a fine idea as well. A couple of the Pokemon that Skeledirge can wall, most notably Iron Valiant, have the ability to go mixed, so that should be accounted for.
Probably the single biggest shut-down to Skeledirge is Chi-Yu. Modest Specs 2HKOs with a resisted Overheat, to say nothing of Dark Pulse, frying all forms of Terastalyzed Skeledirge in the process, while resisting both of Skeledirge's STABs and being immune to Will-O-Wisp. Chi-Yu is an absolute nightmare for Skeledirge's teammates to switch into as well; even Blissey can be 2HKO'd in the sun if Chi-Yu Terastalyzes. Dondozo also doesn't overly care about Will-O-Wisp as it usually runs Rest, ignores Torch Song boosts thanks to its own Unaware, and can wear down Skeledirge with Liquidations and Rest + Sleep Talk. As mentioned before, many other Pokemon have the ability to 0-2HKO it even if they don't switch in as cleanly.