This is likely well known to some, i have talked with MPSers on liberty who had no idea, so posting it!
requires three accounts, and an efficient power character doing the work.
You need the following:
(1) Power character, lvl 33-38 preferably, with the ability to absolutely slaughter MOBs. fire/dev blaster, AR/dev blaster, fire controller, anything that can AoE, repeatedly, and fast
(2) Sidekicker, needs to be 4 levels lower than the (1) Power character optimally, though can be anything up to that level (not as fast, still works really well though)
(2) and (3) go stand together somewhere safe. like the T station in Brickstown.
(2) sidekicks (3)
(1) groups with (3) and is in the same zone obviously
(1) now goes out slaughtering. 6 hours to lvl 20 i extrapolated, from gaining levels 8 to 15 in 2 hours and 45 minutes. with breaks, and a death, and a couple of occasions where he teamed up with other friends to have more fun. its likely possible to do this in less than 6 hours.
why this works:
(3) is sidekicked to within 5 levels of (1)
(1) is fighting heavy duty MOBs at even to +3 levels with relative ease
(3) is therefore getting 1/2 the experience they would normally get for fighting MOBs 5-8 levels above them, including lieuts and bosses, in large quantities with no downtime. its super fast, if you are tired of the low level stuff and really want to try a different sort higher this is a good way to get started.
Note, i assume this will actually work somewhat better at levels over 20, because the relative level cap is +7 for (1) power instead of +5 once you hit 20. this means instead of MOBs optimally 5-8 levels above you, they are 7-10 levels above you, thus alot more exp relatively speaking. probably 20-21 takes less time than 19-20 for example.
wrote this fast, hope its written somewhat legibly.