1~10: Any Rank 1 Behest, not sure if you have to do more than one of these, 2 max.
10~17: Any Rank 10 Behest, repeat until rank 17.
17~23: Rank 20 Behest at Horizon, repeat, repeat, repeat, repeat.
23~33: Look for the rank 30 leves "Necrologos: Adamantine Wills" "Corpus Adamance" "Necrologos: Levinshower" and the rank 20 leve at Horizon "The Devilet Inside". Try to find 8 man groups who have all these, and abort the leves before you finish them (before killing the final mob). You can also do Nanawa Behests at 23~30.
33~50: Continue with those same 30s, adjusting star settings as necessary, behest is mostly dead to you now pick up group wise. At rank 33 you unlock Broken Water leves. Leve cancel until you have Dunesfolk for Dinner, Necrologos: Ranine Reveries, and All Cracked Up. So that's 7 leves you have in a cancelling rotation every reset now. The Devilet Inside can be tossed at rank 35 onward, unless you want to keep it to rank up lower classes.
At rank 42~ you can supplement in between leves with 6 man Raptors in Boulder Downs (North of the Everlakes aetheryte).
Also Efts east of Treespeak work well in the 30~37 range, and the Behest there is rank 40, and always empty (or should be!) so your party can do those every 30 minutes, too.
Fastest way to 50.