I've power levelled several guild members who are tired of missions/quests. They usually get to Asc at L3 or so.
I do it like this (as W/Mo):
Run them from Asc to Sanctum Cay via the usual stops - takes about an hour.
When at Beacons I run them to Droks (sometimes I cannot be bothered cos it's a pain and get them to use a 3k runner
or look for a free 'test' runner).
Once in Droks I take them out to the Troll caves with a number of Scrolls (the best you can afford - get the 10 minute ones in case of foulups). Aggroing and killing all the Trolls with scrolls active levels them up very fast. Don't bother with the bosses they take too long. Ensure you have a good dolyak/cyclone axe build or 55hp monk build.
I then run the Sanctum Cay Mission (no fighting required) then run them to Augury Rock and the Desert Missions (the Oasis to Augury Rock run can be tricky - use a pro for 300g if unsure). Elona I run for them in about 10 mins. Dune s of Despair is a fight - handy if you have used Droks to get them to L15 or so. Thirsty River I solo with hencies and the guildie - bout 75% succes rate depending on spawns - usually within 2 tries.
Then it's just a matter of them ascending and off they go
If they are at L16/17 they should ascend to L20 with the 50k XP.
If you cannot find a guild member:
A runner will do Asc to Lions Arch for about 2k, another from Lions Arch to ToA for about 500 to 1k, then a runner to Sanctum Cay. Some do combos. Runners also do the SC Mission (bout 500g) and the desert missions are about 300g to run to each. I used to run Elona with bonus for 1k each - don't know current prices.
You can find good power levellers on the GWG services board for about 1k per level - they do either Trolls at Droks or Gryffs at Augury Rock.
Edit: Gryffs at Augury Rock levelling has now been nerfed.