well, I played Mu last year, long before k2 came into the scene. I made lvl 84 in about 2 weeks and then practically quit. The only advice I can give you is for Dark Knight. However, I forgot most of the info :<
lvl 1-15 train on the monsters around the main town
lvl 15-30 goto noria (the ice place) and party with some high levels at yeti's. Leech off of them. The exp gain will be good for you and the plvler.
lvl 30-50 goto the dungeon near the town where you start at and kill worms. Then as you near 50, kill cyclops.
50-120 you should try finding a bill of balrog with cyclone ability and also some good armor (scale set i think). Go to the ice place where you trained on yeti's, gather some noobs together, and train. You should get great exp.
120+ goto the lost tower and train.