31-40 Free to Play: Crackrocks, Chafers (-1, -25), or if both are really crowded do Boars, Prespics, Miliboowolf.
Pay to Play: Gob-ball Dungeon or mobs on Tainela, with good Prespic Set is easy to level up there, you can get about 60k Knight online Gold of exp with large group in the dungeon. Use the Piglets Porcos' Territory
If you do rely on air damage then stay at Lousy Pig Knight. You can train on Mushds or Treechnids. Fungi Masters that is alone, which they hardly are Scaraleafs and Trunknids.
Blops are easy to kill and give good xp (beware of the MP loss from Blyph). Dark Smiths, Dark Miners, and Dark Bakers in Bandits' Territory - watch out for Dark Miners' Mass Clumsiness, which reduces AP; Dark Smiths are best for those with ranged attacks
Drunken Panda's are and extremely good (but time consuming) method of obtaining exp, because they usually don't attack. Can normally be found near Akwadala or southern Earth Pandala.
41-60 Free to Play: From now on, Chafers and Crack-rocks are your only source of XP
Pay to Play: Piglets, Treechnids, Miliboowolfs, etc. Blops in Cania Plains, Wabbit Island, Fungi Masters and Scar leafs.
Kanigers with big groups, Lousy Pig Knights, Dark Smiths, Dark Miners, and Dark Bakers in Bandits' Territory. Mushds with prespic sets