Assuming your other characters are really well geared:
1-30: Get Ranger + Druid/Mage DS + Shaman donor buff and run from one end of paludal to the giant mushroom boss. Clear any remaining blue mobs in the general area, then one more clear of the mushroom room/area should get you to 30.
30-40: Head to City of Mist, same buffs and basically repeat the process. At some point (around 40 I think), this method starts to get dangerous.. so....
40-60: In any zone with a good ZEM (again I prefer COM), using a level 70 bard + their well geared cleric bot. The aim is to stack as many DS as possible on the cleric bot. Park bot at lower level char. Have the bard pull as many mobs as possible to the lower level char, and then start playing damage max shield songs. Mobs hit bard, cleric bot heals bard, mobs hit cleric, mobs hit damage shield. With the lower level character, cycle through the mobs and get one hit on each (low level AE spell works well).
60-70: Best way I've found is to have a level 70 dps clear povalor or tipt while the lower char is parked at zone.
Under the right conditions you can do 1-70 in sub 2 hours. I'm sure people have done it in sub-1
