General

Level 10 to 20: Blue Ribbon Pigs/Starfish
Level 20 to 30: Mixed Stone Golems
Level 30 to 40: Sakura Cellions or Mushroom Kingdom
Level 40 to 50: Truckers or Kerning Square
Level 50 to 60: Muddy Banks 3 or Party Quests (Kerning, Moon Bunny, Ludibrium, and Ice Knight)
Level 60 to 70: Sahel 2, Nett's Pyramid, or Monster Park
Level 70 to 80: Sahel 2 or Roids
Level 80 to 110: MP3
Level 80 to 90: Roids
Level 90 to 100: Mr. Allis or Dreamy Ghosts
Level 100 to 110: Captains
Level 110 to 120: Ludibrium
Level 120 to 130: Dragon Turtles
Level 130 to 140: Lycanthropes or Bains at Showa town
Level 140+: Time Temple

Level 110 to 165: Lionheart Castle
Level 165+: Knights Stronghold

That's all I can come up with.

where should I train?

google is ur friend