Best combination for leeching?

I'd recommend getting a x2 card and leveling up to 30 by yourself, these days it takes less than 4 hours. After that, if it's possible, you could rig CPQ with some friends to give yourself exp from winning until you can solo one side of rooms 3 or 4 with max speed and trojans. I can say I've hit 50 in under 24 hours (Total play time over 2-3 days) on my Pirate, Aran and Evan with this method. After you hit 51, you could try to rig a CPQ2, or just leech yourself, although I wouldn't know what to do that on.