This is a list of things you may want to do before moving to the next significant range of levels and also where to level up
Level 30-40 Range
Leveling
Soloing can be done on
If you are doing XP as a group these are the areas that you'll want to be.
Farming
NPC Quests
1-7B Ignorance Upgrader for level 35 weapons
Level 40-50 Range
Leveling
Soloing can be done on ABAs, Calianas pretty reasonably up to about 47 or 48. This is a good idea if you enjoy grinding and it will make sure you have plenty money for later
Soloing to try and just become higher level can be done in VC solo. Grumbles are red at 40 and are great all the way up to 45. After that you can go to elan and xp on the tiny scuds at the entrance as well as the big dracos. Even if they hit you hard, if you deal decent damage, they will get stuck in a healing cycle and heal themselves usually after 2 attacks.
If you are doing XP as a group these are the areas that you'll want to be.
40-45 VC grumbles, ash, volides near the belphegor entrance
45-49 VC babas and dracos
49-50 Elan turncoats at TC3. There are lower elan turncoats and mobs but generally as a party you can get better XP in VC until you have someone to be able to get the killing hits on TC3's.
Farming
Farming calianas at 47 is generally accepted as the best level to farm them. You get lots of loot, can wear the best armor possible(until 50) and can eventually get C type 50 weapons that allow you to farm huge amounts of these at a time.
Farming turncoats in sette is generally best to do while leveling yourself up, if you start at around 37 and level there til you're 41 or 42, you should have most of your spells if not all of them. You can do this to help other players if you aren't a mage. The booty and spells NPC for a pretty good amount too
Miscellaneous
Leon BD keys
Fixing all your PT's
Level 50-55 Range
Leveling
Soloing can be done on Red Haired Splinters if you are a level 51+ melee with 5k pots and good armor or block. The monster debuffs you with hell bless, attacks you, and then refreshes itself. So only 1 out of the 3 actions it performs are an attack.
If you are doing XP as a group these are the areas that you'll want to be.
50-51 Elan ABX Pit
51-53 BM/OCL Red Haired Splinters and random monsters while waiting respawn
53-55 BM Lizards or Cartella Turncoats
Farming
Cartella reapers and Izen builders
VC Currs
Miscellaneous
Leon BD keys - they also have a chance to give type B 53/55 armors, type B 55 weapons, and type C 55 weapons and armor with level 50 requirement.
If you have a group you can do Hora Dark Holes for a chance of getting the best accessories in the game
Major PB's in Elan, BM, OCL, Cartella. The unique weapons have good effects and damage.
Regular PB's to help you save up talics for your end-game 55 superior armor pieces.