There is no point in any L20 build (ie, 1 barb/19 sorc or 20 sorc) since the most efficient way to get to L20 is to hold levels at L18 which lets you get full XP out of more quests.
There is probably no way to actually level a 3d life in 24 clock hours. There is every possibility of leveling a 3d life in 24 quest hours. The problem is time lost in getting groups together, buffing, traveling to the quest, stopping for bio, etc.
Add to that the restrictions of no friends, guild, minimal gear, etc. and it becomes more of an issue.
As far as strategy, there is really little reason to hold levels until L18. With a bit of planning you can select quests and run them the optimal number of times to get most XP and never have to worry about being above level.
Also, always maintain the elite streak. On a 3d life you can open elite directly so should always do that first time thru.
IMO greater tomes of learning and XP potions make it a lot easier. Any buff to XP that you can get is a buff you should get. Run on an XP weekend if possible.
Certainly doable if run Friday thru Sunday. But, in 24 clock hours -- that would be a lot of work and I'm not sure it could actually be done unless you had a preset group.