So. Ghost/Fighting, as anyone who's used Marshadow will know, was described as "perfect neutral coverage" back then; nowadays that's no longer the case with H-Zoroark running around, but it's for that reason that Decidueye has other moves as well. It will happily Brave Bird the Zoroark and get a decent amount of damage out of it. So clearly, nothing can resist all the moves, but I was wondering how close it was possible to get.
Brave Bird, Leaf Blade, and Spirit Shackle all have some kind of STAB boost, so just looking at those three moves, already we're down to just one existing type combo that resists all three: Steel/Dark, represented by the Kingambit line and nothing else. And of course, that fourth move takes advantage of a huge weakness and Kingambit's substantial weight--even if you make it as physically bulky as possible, Low Kick is still a 2HKO. Eviolite Bisharp fares a bit better, but still takes way too much for comfort.
Enter Pawniard. Yes, first-stage, 340-BST Pawniard. It's light enough that Low Kick is only 40 power, so Eviolite would effectively reduce that to 26 power. Or, alternatively, since it's just above the lowest jump point, you can use Float Stone to kick it back down to 5.01 and make Low Kick only 20 power, though that won't reduce any damage on the other moves. As it turns out, the resisted Brave Bird is right around the same damage as a double-weakness Low Kick, which should be encouraged since every time it picks Brave Bird that's a bit of chip damage to wear it down, and it's less than 30% in any event.
After experimenting with this, one thing became clear. Because of Decidueye's Brave nature, Pawniard only needs 12 Speed EVs to creep it, and you should absolutely go for that since it means you get Iron Defense up before their move, and now suddenly 30% turns to 15%.
Unfortunately, Pawniard's move pool is rather lacking. It gets about one good support move (Metal Sound) for enabling Miraidon sweeps, and that's it. If the shield ever goes up, Pawniard can max out on Iron Defenses and focus on not dying, using cheers as necessary, but that's about it. It gets Foul Play to synergize with Decidueye's own Swords Dance, and Poison Jab to fish for even more chip damage on every remaining turn, ultimately nothing impressive from either attack though. I've managed to get about halfway through the health bar on a solo effort (to include a shield break after the low-timer warning), but full soloing with this is probably not feasible. If it were, the moveset for it is likely Iron Defense/Swords Dance/Iron Head/Rest with Tera Steel, figuring that it'll switch to Spirit Shackle on most turns after the tera is revealed, which means less chip from Brave Bird.