Ruft pointed out to me earlier today that choice specs don't boost doom desire if Jirachi switches out, unfortunately. However, I do like the pressure doom desire generates, so I have been using the following set to some success:
Jirachi @ Expert Belt
Ability: Serene Grace
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Doom Desire
- Psychic
- Thunder
- Grass Knot
First of all, a few calcs to show the type of damage this set does:
252+ SpA Jirachi Doom Desire vs. 0 HP / 4 SpD Dragapult: 264-312 (83.2 - 98.4%) -- 68.8% chance to OHKO after Stealth Rock
252+ SpA Expert Belt Jirachi Doom Desire vs. 252 HP / 252+ SpD Clefable: 389-461 (98.7 - 117%) -- 93.8% chance to OHKO after Leftovers recovery
252+ SpA Expert Belt Jirachi Psychic vs. 252 HP / 4 SpD Toxapex: 240-283 (78.9 - 93%) -- 43.8% chance to OHKO after Stealth Rock and Black Sludge recovery
252+ SpA Expert Belt Jirachi Psychic vs. 252 HP / 0 SpD Kommo-o: 310-367 (87.5 - 103.6%) -- 56.3% chance to OHKO after Stealth Rock and Leftovers recovery
252+ SpA Expert Belt Jirachi Psychic vs. 0 HP / 4 SpD Keldeo: 353-418 (109.2 - 129.4%) -- guaranteed OHKO
252+ SpA Expert Belt Jirachi Grass Knot (80 BP) vs. 252 HP / 4 SpD Seismitoad: 485-571 (117.1 - 137.9%) -- guaranteed OHKO
Thunder also 2HKO's Corviknight, Mandibuzz, and Toxapex + has a 60% chance to para (when it hits lol)
Doom desire is very powerful for the pressure it creates; I pair this set with Rotom-H which can scare out the steel types and water types that the opponent wants to have take the doom desire, although as the calcs above show Jirachi can deal with some of those itself. It also pairs well with phazers like Hippowdon and Kommo-o which can force the opponent out into something that dies to doom desire. Additionally, since my team has a Dracovish, Grass Knot is very useful to take out Seismitoad, and it also deals a ton to Hippowdon. However, depending on your team needs, there are a ton of moves you can use besides Thunder and Grass Knot. Aura Sphere, U-turn, Healing Wish, and CM are all reasonably good options. Psychic is preferred over Psyshock since it deals significantly more damage to Toxapex, Conk, and Kommo-o.
Modest max speed is fast enough to outspend the Rotoms; since you're not trying to flinch anything to death, I think it's better to go modest and maximize your damage output. You could probably transfer some of the speed to bulk if you wanted. I tried the set with leftovers but I think the extra damage from Expert Belt is significantly more useful.
A few replays to show its uses:
https://replay.pokemonshowdown.com/gen8ou-1088776683 - Double to Jirachi on turn 6 on the obvious Seismitoad and then OHKO it with Grass Knot, opening the door for Dracovish. Then on turn 11, he can't switch to his Hydreigon because it would die to Fishious Rend + Doom Desire, meaning his Chandelure that threatened to break through my team has to die.
https://replay.pokemonshowdown.com/gen8ou-1088704526 - Jirachi puts his Hippowdon and Corviknight out of commission, opening the door for Excadrill and Dracovish to win. Also I would have won a lot faster if I had just overheated on turn 10...
https://replay.pokemonshowdown.com/gen8ou-1088686028 - Jirachi takes out Ferrothorn and Toxapex, again taking away switch-ins to Dracovish.
https://replay.pokemonshowdown.com/gen8ou-1088684305 - Jirachi kills his only Dracovish switch-in turn 1
https://replay.pokemonshowdown.com/gen8ou-1088591870 - Weakens Toxapex and kills Excadrill in the first 3 turns.
https://replay.pokemonshowdown.com/gen8ou-1088572682 - Hippowdon whirlwinds in Clef on the Doom Desire, bringing it really low.
https://replay.pokemonshowdown.com/gen8ou-1088885376 - Another game where the opponent's only answer to Dracovish (Seismitoad, as usual) gets taken out by Jirachi.
There were more games where Jirachi put in work and I didn't save the replay, but hopefully this demonstrates the power of this set and how it outputs a lot of pressure against all kinds of teams.