I'm just doing this to help my guild and others, it may not all be correct so if something isn't accurate reply me something better.
1-20 Do your missions.
20-30 2nd job mission. If cashed grind Gaikoz and do missions.
Alt. 25-30 Ellia Continent.
30-40 Lower Floor Kastulle. (Recommended because Upper Floor is very hard, to me anyway)
Alt. 35-40 Battle for Bermishia (If you can find a good party)
40-50 Battle for Bermishia (Recommended)
Alt. 40-50+ Wyrms (Most rooms at Wyrms are 50+)
Also, if your great at PvP like a few friends of mine then PvP would be your favorite and best way of leveling.
Enjoy! Happy grinding.