Solo up to 15ish
Behest at (popular R20 camp) with supplementary solo/leves. Until 24ish
Behest at (popular R30 camp) with supplementary solo/leves. Until 34ish *
Behest at (popular R40 camp) until 44ish. *

Where * is, see Aera's post above about leves. More or less.
Failing the desired leves that give great SP is the fastest way to rank up, at the cost of losing out on rewards.