Aren't standard Jirachi's EVed to about 271 Speed to outspeed Choice Band Heracross and all DD Mence? If then, Jirachi will survive and the Wish will still activate.
don't forget the possibility of getting flinched by Zen Headbutt.
But anyways, this set already exists and it is very deadly, but it really requires Wish support to work at full extent. Unlike Garchomp, however, this thing is not Sandstorm friendly and is weak to Stealth Rock (although immunity to Spikes and having useful resistances/EQ immunity and Intimidate compensates this in a way). Also the effectiveness of this set depends on the usage of Cresselia, which is increasing btw just because Chain Chomp is invented and everyone's using "GyaraVire".
Prediction is key to using this guy. I don't need to explain it since people have done so (and btw this set already exists and SePh used it quite a while ago).
Jirachi case:
Assuming Jirachi has 271 speed (some people might opt for this due to the fact a lot of stuff runs at 270)
Jirachi switches in Meteor, takes damage
Jirachi Wishes
Mence Fire Blasts
Jirachi does something not called Ice Punch
Mence Fire Blast
Jirachi's dead.
Wish failed.
However, let's look at this
The chances of Jirachi flinching: 36% (90% acc Zen Headbutt)
The chances of Salamence successfully hitting 3x (Draco, Fire Blast, Fire Blast): 65%.
If my math is correct, 65 x 64% = 41.6% chance of successfully killing Jirachi.
Cresselia still counters this btw