One I've been fiddling with:
Jirachi
@ Life Orb/Leftovers
Serene Grace
EVs: 252 HP/ 4 SpA/ 252 Spe
Timid
Move Set:
Cosmic Power
Rest
Charge Beam
Flash Cannon/Psychic
Comment: With Cosmic Power and Rest, Jirachi will last a verrrrry long time and has no problem getting multiple boosts. Thanks to Charge Beam, when Jirachi is happy being unkillable, the offense boosting can begin and will happen quickly.
Now, there's a lot of alternatives here. If you have a lot of paralysis support, you can max Special Attack instead of speed. Leftovers is almost overkill but it's nice to not have to rest as often, though with 6 CPs, it's pretty easy to get away with.
The strongest attack you'll have to watch for is a CB Mamo's EQ which rarely OHKOs after a boost (without absolute max attack), even with Stealth Rock while you can nail it with Flash Cannon for an easy OHKO, with Life Orb and no investments and Stealth Rock.
With two boosts, even ScarfTran's Fire Blast won't 2HKO, but Jirachi can't do much back.
This thing is possibly the best final pokemon to have as even Blissey will eventually fall thanks to SpD drops and immunity to status.
I don't know if the SubCM set can work better, but this one is far more defensive, capable of fending off Earthquakes (all but the strongest) and capable of outstalling everything thanks to Rest.
Some comments:
1) Charge Beam as your only coverage move leaves Jirachi in a tight spot against a lot of Pokemon.
2) You probably want to run at least 80 Special Attack EVs like the SubCM Jirachi, which allows you to take blissey better once you start boosting your attack.
3) Wish may be preferable to rest, as you can alternate wishes and boosting moves so that you waste less time asleep. You lose the ability to shrug off thunderwaves, but most blissey pack Toxic anyways.