Challenge log group and NM farming groups generally don't look for people of a certain level requirement so I'd shout something like, "Lvl 21 <class> LFG - Challenge Log/NM Farming". Make sure to turn on the Seeking Party option also in the party options. The problem is most people are now level 35 and farming light on Dragons which you generally need a full lvl 35 party to keep the chain going to 30.