Specs STAB Modest Choice Specs Thunder against +2 252/0 Jirachi does ~40%. That's definitelty going to 3hko after hazards. You could argue you would miss, but Sub CM Jirachi is always used under rain so that it can use Thunder, which has 3x the para chance of Thunderbolt, so your Thunder's will be 100% accurate too. You also have LO & rain boosted Surf, and Gengar's special move. In the end you just need to break its Sub and make sure it's within KO range of CC and you'll beat it only losing one mon :)
I didn't see you on PS at our earlier, we have were supposed to have a lesson at 12pm, lol. If you still want a lesson, just reply to this vm and get on PS :)