Gastrodon @ Leftovers
252 HP/4 SpA/252 SpD
Calm Nature (+SpD, - Atk)
Trait: Storm Drain
-Recover
-Earth Power
-Ice Beam
-Toxic
Gastrodon just doesn't care about Slowbro and Jirachi at all (unless Jirachi gets 8 or 9 flinches in a row with Iron Head, around a 1% chance). On the flip side, it cripples Slowbro with Toxic and does around 50% to Jirachi with Earth Power. Landorus can put a decent dent in Gastrodon with Earth Power, but it's almost never a 2HKO after Stealth Rock, while Gastrodon does 96% at minimum to Landorus with Ice Beam. Gastrodon also has Recover, making it very hard to wear down. As for the EVs, they're poured in to special bulk to take hits from Landorus as well as possible.
There are a few other options for attacks, mainly Counter, Mirror Coat, Earthquake, Surf and Scald. These weren't chosen because they don't have any utility against the current team 1. Surf/Scald can't OHKO Landorus and Earthquake does as much as Earth Power to Jirachi, while the former two aren't that great if the opponent knows they're there. If you disagree with my attack and EV choices please say so.
A very similar choice has been nominated, and that's Seismitoad. The two have very similar stats, Gastrodon has Recover and Ice Beam over Seismitoad, while Seismitoad has Stealth Rock. Gastrodon is probably a better pick because Seismitoad tends to get 2HKOed by Landorus after Stealth Rock even with maximum SpD investment,
and can't even OHKO it in turn. It also gets worn down very easily.
Stealth Rock doesn't seem desparately important on a member at this point, but that is Seismitoad's advantage over Gastrodon.
As for Gastrodon's disadvantages, it certainly has them. Grass-types are public enemy number one. In particular, Ferrothorn has no trouble setting up hazards on Gastrodon and Celebi can be customized to do whatever it wants without ever caring about Gastrodon. Landorus' U-turn makes Grass-types even more annoying, as it can keep the offensive momentum on a Gastrodon switch-in. If Gastrodon is selected, something that can put pressure on Celebi would be a good fourth choice.